If you can make it for 30 minutes, then come for 30 minutes. If you can stay the whole time, then stay the whole time.
We all have our lives to live. The times for the events will show you when I will be there, so if you pop in at any time during that time slot, I'd be delighted to see you.
I've had people coming for 10 minutes, all the way to staying 1 and a half hours after the end time to continue conversations with people.
Its flexible, so don't stress!