It varies depending on the venue, how big the pit is, how big the orchestra is, and how far they have to walk - but I generally do the following (UK here as well):
7 minutes before
"Ladies and Gentlemen of the Orchestra, this is your first call to the pit. Your first call please, Ladies and Gentlemen of the Orchestra."
5 minutes before (with beginners)
"Ladies and Gentlemen of the Orchestra, this is your second and final call to the pit. Your second and final call. Your call please, (Conductor) - (Conductor), this is your call."
Unless it's a really short interval, I'll do the same before a restart as well.
Hope that helps!
Cx