When to replace tires?

[Big.Z.hummin]

So I know this is a preference question but i'm wondering on average when is a good time to replace tires. I have toyo m/t's with almost 40k miles on them and they appear to have 35 to 40% tread on them. There is obviously a good amount of tread left just seeing what you guys think about it. no telling when i have the urge to do a little offroading and i don't want my lack of tread to keep the beast from plugging along.