There is a leak in the walls and a crack in pipe and found thick black mold. How long has the leak been going on? Months or years? I need to determine how long there's been water leaking into the walls... what clues could I look for?

Why does it matter how long as nobody including you knew it was leaking until recently. (If you are planning on suing someone you have to prove they KNEW about it and did nothing)

Just that the correct repairs need to be done now should be you only concern.

Wood rot, mineral deposits at the lowest spot of dripping... but various factors go into determining for how long.

I agree that suing is going to be difficult. Not only amount of time gone by, but also how detectable a defect is. An inspector isn't going to rip walls out.
The whole subject of mold is overblown. Most mold isn't toxic. Most mold is black. Rip it out or bleach it.

I think you will need to get the wood tested to get an approximate response, otherwise focus on fixing it and send the landlord a bill.

I think you will need to get the wood tested to get an approximate response, otherwise focus on fixing it and send the landlord a bill.
Where did they mention landlord? If you go ripping up walls as a renter you would open yourself up for a lot of trouble and expense. And as a renter most places... you don't get the choice to get someone to repair it and send a bill to your landlord. You HAVE to call the landlord first. Unless they gave approval to do so in advance (unlikely because they would have preferred people if not themselves do it) a renter would almost certainly be stuck paying the bill without reimbursement.

Based on everything said by the OP. Its likely a house they own, not rent.

Why?

If you need something exact, like for court, your opinion and your clues mean nothing, and you would need an expert to come in and investigate and give you a written report.

Also other issues of climate, is it in a tropical area? Was there a prior leak, that was fixed and a new leak developed and so on.