Another point is that 10mb is a lot of text, especially for users like me who use these types of note-taking apps as todo lists. For persistent notes I already have directory I store locally w/ cloud backup, but a webapp would be nice for my running todo list.
That will hopefully never top 10mb,so a free user like me would really eat into hosting costs. Perhaps a solution to people like the OP who are concerned about data being held hostage is to make it clear that after the trial period their data can be exported, that way they don't have to feel there is risk of it being held hostage.
In my note taking "system" (vim on a text file inside a GNU screen session) for work, I think I've only hit 1MB total for the last 10 years.
If the OP is going to do a time-unlimited demo, it should be really small, like 5K or less. The point is the same: try it out for a little while, and see if it works for you.
On the other hand 10mb is not a lot of images. If I'm using a paid mobile app for note-taking, I probably want to be able to quickly snap a photo and maybe write a few bullet points or sentences.
According to https://inkdrop.app/pricing the free plan includes 1 GB storage, and the basic plan ($5/mo) includes 10 GB storage.
I agree with sibling comments that a limit based on number of notes would be preferable to losing access after a specified time. Maybe adjust the free tier storage allocation down, or limit the number of images/attachments.
That will hopefully never top 10mb,so a free user like me would really eat into hosting costs. Perhaps a solution to people like the OP who are concerned about data being held hostage is to make it clear that after the trial period their data can be exported, that way they don't have to feel there is risk of it being held hostage.