This is for the same app you were praising for being ad-free in one of your other threads? Just curious, since you have posted to the forum that was created for that app in response to your post.
I'm not an android developer, but there are rules about what types of ads are allowed, which should be documented by Google if you are uploading to the Play Store. The trickier thing to judge is what your users will tolerate (as discussed above). BTW I would immediately delete any app that ever showed me a 30 second ad (no second chances, it would be deleted there and then, no matter what the app was).
Since this particular app is a VPN, users will probably not be interacting with its interface very often, more likely they just set it up and leave it running. In which case they will only see your ads on the occasions when they interact with it, which I would think would limit ad revenue. I'm pretty certain that the Play Store forbids any app from displaying ads other than when the app is open on the user's screen (something they eventually banned after months of outcry about apps that did just that). Apps that did that were regarded as malware by almost all users, and there was a time when a common question here was how to identify the app that was doing this so that the user could delete it.