Podcasts used to be ad-light oases. Not anymore.

Ads in the second quarter of this year took up an average of 10.9% of podcast run times, up from 7.9% in the second quarter of 2021, according to new research from marketing agency Oxford Road and audio measurement company Podscribe.

In 2024, podcasters collected 6 cents in ad revenue per hour of programming that was heard by a listener, an increase from just under 5 cents in 2021 and 2 cents in 2015.

To some, the numbers represent a win for podcasting—a medium that has steadily increased its audience over the past two decades, offering a more intimate experience than other media, and which only recently convinced large multinational brands to become advertisers.

But others worry the industry is getting too greedy in its race to monetize and might wind up killing value for advertisers and the intimate experience for listeners in the process.

“Some publishers have already pushed their ad load beyond where it should be," said Dan Granger, the chief executive and founder of Oxford Road, which helps marketers create and buy time for podcast ads. “This is not going in a direction that’s going to end well for any of us."

The first podcasts to sell ads would often place one commercial at the beginning of their show, and another in the middle. Now it isn’t uncommon for podcasts to run ads after every 10 minutes, stack two or three ads per commercial break and play even more at the end of each episode. A recent hourlong episode of Alex Cooper’s popular “Call Her Daddy" podcast, for example, included eight ads over four slots.

The increasing commercial time per episode—known in the industry as ad load—has been driven by both supply and demand trends, analysts say.

The first podcast advertisers tended to be smaller, direct-to-consumer companies that couldn’t afford the cost of TV advertising, with podcasters brokering the deals and scripting and reading the ads themselves. Household brands with big budgets began joining the medium as listener figures grew and new podcast networks and automated bidding made it easier for marketers to buy time on a large scale.

The industry also faced pressure to start making more cash.

Audio streaming giant Spotify spent more than $1 billion buying up podcast studios such as Gimlet and signing deals with the likes of former President Barack Obama, but it struggled to turn a profit on the investments and cut staff. Many other podcasting companies and divisions, including Malcolm Gladwell’s Pushkin Industries, NPR and New York Public Radio, last year made layoffs in what Slate described as a “podcasting bloodbath" following a lackluster few quarters in digital advertising. And podcast production company Paradiso Media filed for bankruptcy this July.

“You saw the markets tightening up while there were these great obligations to pay for Meghan Markle and Kim Kardashian," Granger said. “You can’t just force the advertisers to start paying premiums because the business wasn’t moving that way, and so they were kind of forced to create new avenues to maximize their yield, and as such, we have a higher ad load."

Finding the skip button

To be sure, podcasts’ ad load is still relatively low compared with almost every other type of media, according to Emarketer figures. And some analysts think the most popular podcasts could stand to carry even more ads, given U.S. consumers’ historic capacity to endure commercial breaks on TV and radio. Only 10% of podcast listeners describe podcast ads as intolerable or barely tolerable, while 42% think they are “slightly annoying, but tolerable," according to data from SoundsProfitable, a podcasting research and advocacy group.

But other podcasting executives think that adding more ads to podcast episodes would backfire. Research from Oxford Road and Podscribe found that the likelihood of a listener responding to an ad—by visiting an advertiser’s website or making a purchase, for example—decreases as the number of ads per episode goes up.

A listener might not bother to skip a single ad but might be more inclined to do so when faced with two minutes of ads in one go, said Pete Birsinger, the founder and chief executive of Podscribe.

Claire Parker and Ashley Hamilton, the comedians behind the podcast “Celebrity Memoir Book Club," said they tried numerous podcast advertising networks to find a partner that doesn’t pressure them to load their episodes with as many commercials as possible, particularly produced ads from companies they don’t endorse themselves.

“We would rather work with fewer brands that we truly believe in and can sell to you successfully, as opposed to having these cheapened background noise ads that people are fast forwarding through," Parker said.

In addition to selling ads through the Vox Media network, where the hosts eventually landed, they make money from hosting shows in front of live audiences and running a Patreon account, which charges fans $5 or $10 a month in exchange for bonus episodes and other perks.

Defector, an employee-owned sports and culture media company that makes podcasts such as “The Distraction" and “Normal Gossip," has opened its shows to produced ads sold through automated platforms, but it is more concerned about the quality of ads than the quantity, according to Jasper Wang, vice president of revenue and operations.

“If you can feel like you’re curating the right set of sponsors, then maybe you do have a little bit more of a leeway with your audience on adding a slot here and there," he said.

Write to Katie Deighton at katie.deighton@wsj.com