I quit my job.. How long after does an employer have to file criminal theft charges against me? And what kind of evidence would they need to file the charges?

Where? The employer doesn't file. He reports the theft to the Police and then it goes from there.

Statute depends on where and how much.

They need proof - or what they think is proof. Video, money missing, proof of receipts but the money isn't in the register, depends on how the stealing was done.

I assume you did steal and are concerned?

They just call the police and report it, the SOL varies on first how much money was stolen, a felony may have a SOL from 1 to 3 years that they have to charge you. ( even longer in some places)

The amount of evidence varies by each DA and if they think they have a case.