How long is too long for rat's teeth?

I was having a photo shoot with Tia today (pics coming soon, when i can actually work out how everything works on the new camera:rolleyes: ) and i noticed her grinding her teeth. I know that rats chatter their teeth when they're content and Tia does that often, but this time she was distinctly grinding them, a sort of side-to-side motion. I had a look in her mouth (not easy if you know how squirmy rats can be!) and concentrated mainly on her bottom incisors. They don't look too long as they aren't curving inwards or anything, and they're a nice healthy yellowish colour (yellow is the colour of rat enamel, so it's normal), but i noticed that they're beginning to separate slightly at the top. Was this why she was grinding? She has a wood chew but hardly ever uses it, so i expected that her teeth would get long and sooner or later would have to be clipped. But she doesn't show signs of discomfort when eating, but then i thought that it shouldn't get that far before you have them clipped anyway. My mum had a look, but she said that if we took her and it turned out that they weren't too long, it would be a waste of £10 to just see a vet. It's difficult to tell, but i think that her lower incisors reach just past the top of her upper ones and end on that gum. They'll need to be clipped sooner or later, but is now the right time?