When you get an IoP you have to set 2 openings for base raids. So I'm assuing twice a week (unless you keep changing the window).
Not sure if getting more IoP's = more raid windows though.
I expect that raid window will be set for at least the upcoming week so people wont keep shifting the schedule so its impossible to raid them 
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
I think the actual raid has to be scheduled 3 days in advance, once set it's on (unless the attackers don't show of course)
i thought it was 2? ah well, at least an X number of days
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
It could be two, I only have a vague memory of seeing a post somewhere.
Base raiding - how frequently can this be done?
ie, can you be raided weekly, monthly, only in a specific timeframe?