When the app runs, it check it's profile. If the ad-hoc profile on the device expires, the app will look online for a fresh one. Expired profiles stay on the device until deleted.
About an ad-hoc profile being good for one year..that's max - the app won't function beyond the one year annual period for the account they are made under. If your dev account expires in May, an ad-hoc profile created today won't work past that date. If you renew the account, you'll need to renew distribution/ad-hoc certs, and generate new profiles against them.
You really are the best! Always see your responses on forums and glad you picked mine up!
So just on renewing distribution/ ad-hoc certs and new profiles against them, does this mean that if for example I create an ad-hoc profile today and my account expires in august, can I create a new cert, and profile after this and have the app still work without buildin the app from Xcode to the device? Or will all apps need a fresh build every year on account renewal?
>fresh build every year on account renewal?
When an app is signed/built/archived as an ipa w/xode for ad-hoc distribution, it is signed using ad-hoc (mobile) provisioning, against a specific developer certificate.
If the app was initially signed/built/archived against a now expired dev cert, a fresh/new profile will not allow it to run. The app in that case needs to be re-signed, rebuilt and re-distributed, along with a new mobile provisioning profile.
BTW, I believe the assumption is that one year is usually beyond the typically frequent need to distribute fresh builds during testing. If there are known long delays, I'd suggest to wait on creating a build to be distributed until the testers are ready. I'd not recommend to use ad-hoc as a means of avoiding Apple's approved distribution methods just to be able to casually install apps to selected devices long term, of which there is no approved method.
See: h ttps://coderwall.com/p/r5jzzw/creating-an-itunes-ipa-file-for-ad-hoc-distribution-in-30-steps-xcode-for-ios
Note the ad-hoc distribution process discussed above is for a Developer Program account, and not to be confused w/a Developer Enterprise Program account inhouse distribution process.