We used the v-room last July. It was brilliant. You can come and go as often as you like but you probably won't want too. It has a lovely play area for toddlers, an area for teens and an adults only cafe bar. Our flight was delayed by 2 hours and we were allowed to stay there until our flight was boarding. It does close when the last flight has been called.
In theory, there's a time limit (albeit I can't remember what it is), but in reality it's unlikely to be imposed. As RDP's experience shows, you're not likely to be turfed out if your flight's delayed.