is it random how long they last? i did 3 sc games which took 1 hour to get enough points to buy a tool and went mining and it lasted 14k!

The tools last a set amount of charge. The rate that this charge decays is constant for any skill but differs between skills. For any particular skill, there's an amount of exp you get per tool. If your tool is volatile, resetting it also lowers this charge.

it said it disentegrated, so its done now?

Yes.

 

If you're going for sc tools I'd suggest using a non-combat clan, it's around 3 tools/hour.

 

I'm assuming you're using picks to speed up mining til 77/80? They greatly reduce the annoyance of power mining iron/granite, but they're technically slower overall.

 

Edit:

 

Here's a nice table of the exp each tool gives