I'm in the UK too. We have to apply (and pay our fee) to NISA directly, stating which rink we want to test at. They then forwards the test papers to the rink organiser(s). When there are enough test papers to hold a test the organiser can then set a date. Any changes of rinks are arranged by the 2 rink's organisers. Test papers once submitted are only valid for a year.
Free and Field movesAt my rink these have the same organiser and there are lots of people wanting to test these so tests end up being held every 2-3 months and are 5-6hrs long. There is sometimes a waiting list but if you don't get into one, you will the next.
Dance testsWe have a different organiser for this. Tests are arranged as soon as we meet NISA's minimum numbers rules which is usually once or twice a year. Our dance coaches try to work out between them who will need testing and when we will be likely to have enough people to hold a test. Skaters at rink ~hr away sometimes are transferred to here by their organiser in order to make up enough numbers which sometimes triggers tests earlier than expected by our coaches.
We only get a couple of weeks notice unless a judge isn't local (eg for high level tests).
Some organisers across Scotland keep in touch in order to arrange things like high level tests (which need 2 judges) in a block, so an extra judge can be brought in from down south to travel round the rinks over a couple of days. (This happened in December here.)
We've had 4 tests since summer 2011 when I started dance, though really only 3 of those were "ours". The latest one was held here because a rink ~3hrs north had to cancel their test date and they couldn't get ice time in time for the kids to pass their levels so they could meet the Solo dance series criteria. We had ice time! (The series starts early March). It was held 29 days after our December test so one of our skaters could join in too and re-take the test she needed for it too!
Who knows when our next dance test will be!