I thought you were supposed to call them when it all starts, and then actually go to the hospital when they are 5 minutes apart. That's what I plan to do anyway. I think there is no harm in calling. Hope things are all going well for you OP, maybe you've even had the baby by now!!
I was told last time (and this time) to call when they start so that the hospital are aware, they'll tell you when they want you to go in and how often they want updates. I was told to stay at home until they were five minutes apart unless my waters broke, I wasn't coping, or I had any bleeding. They also told me to ring them with an update roughly every 4-5 hours and after the third update they asked me to go in for a quick CTG just to check baby was happy.
How soon they want you to go in will depend on different things like your pregnancy history, how this pregnancy has been, how far you live from the hospital and the time of day (my unit has told me that if it's rush hour they want me there ASAP as the trip can take around an hour in heavy traffic, more if there's roadworks or accidents), etc.
Just give them a quick ring, they'll let you know what's what.