I'll be updating the time limits in this post shortly but to help answer your question, all of the VMs have some form of time limit just the experience varies. If you're able to take a snapshot before Virtualbox launches the VM, then you may be able to revert back to that VM when one hits the time limit. The exceptions are the WinXP/IE6 & Win8/IE10 images which expire after 90 days from the time we upload them. In that case, a snapshot won't help you out.

Again, I'll be updating the post next week with more details about the time limits.