What to charge

I’m an independant consultant (if you could call it that) and lately I’ve been finding that a lot of jobs I take, take a lot longer than I originally thought they should. They all have involved hsving to correct other problems before I can correct the problem I was called in to repair.

I usually state an upfront charge based on how long I think it will take to correct the problem but then It takes much longer due to the unforeseen circumstances.

I’m wondering what I should do in these cases, charge for the entire time I’m there or for the anticipated work that I stated up front? Or just change my wording to something that reflects the fact that there could be a fair bit extra time and therefore money involved?