Can I use the assets I licensed during the free trial after it ends?

Check here for the licensing terms: https://stock.adobe.com/license-terms

As @Jill_C stated, licences are perpetual and don't expire in time (however, other conditions may make them expire, like the print run).

I would also point out, that you should download the assets to your computer. You can redownload the licensed asset for free multiple times, but it happens that assets get taken down, either by Adobe or by the contributor. In those cases, you won't be able to access the file again from the stock site. But you are still allowed to use the asset, except in the rare case where Adobe will inform you that the licence has been revoked. I'm not aware of this having ever happened, however.

Even after the subscription ends, you can still licence the free assets. If you have after the cancel a need for stock assets, you won't be able to take the free subscription again. You will need in that case to use any of the for-pay options.

I should clarify my earlier statement. You CAN use those images for multiple customers, but you need to purchase a license for each one individually.