Normally the Market Application will let the user that updates are available for one or more of their installed applications via a Status Bar notification, but users aren t forced to install the update.

I think stopping the application working after a time period, forcing to the user to install a new update is not a good thing to do because:

• A user might not want to update your application if it s working. Updating a lot of applications via the Market can be chore due to the interminable prompts so I sometimes don t get round to updating applications as soon as updates are available.
• It forces you to release possibly needless updates when the previous version is about to time out.
• It s not very nice to the users.

Why do you want to force updates on users? If you explain the problem you are trying to solve by doing this someone might be able to give you a better solution.

The only thing you can do to make it more likely that users will upgrading update the description of your application in the Market to include the benefits of upgrading, as users can read the update description before deciding whether to upgrade.

I have heard about notifications sometimes working and sometimes not. My users tell me they are NOT notified when I update my application but that when they view their "My Downloads" from the Market application on their phone, it shows "Update Available" next to the application. If they do not go into "My Downloads" they never know an update is available.

"Or should I be writing code in my apps to stop working after a fixed
 period thus forcing the user to download the new version?"

I would not recommend this unless the application will no longer work w/o the update. The Android Market is so fickle that sometimes applications dissapear from "My Downloads" when a carrier updates their system software on your user s phone. If that happens and your application expires your users will not be able to access the application until Google fixes the problem which can take a week or more.

You can check for new versions on your server and then programmatically direct them to the market place:

startActivity(new Intent(Intent.ACTION_VIEW, Uri.parse("market://details?id=com.yourapp...")));