Job overpaid me after I quit- sent them check to repay- how long do they have to cash it?

I recently quit my job with appropriate notice and my employer did not take me off payroll right away. I ended up with a couple extra paychecks. I finally got their attention to stop paying me, and then sen them a check for amount they paid me

I deducted the expenses for unused benefits since it's their fault they were paid. It seems very fair to me.

However, it's been approximately 4 months since they received the check and they have not cashed it. How long do they have to cash the check? (I know the bank expires them in 6 mos, but what does the law state for recovery?)

They also seem to have broken a couple laws by not sending my final check without the alotted amout of time, and denied my my final 3-4 paystubs when requested.

I hate for this to drag on because now I'm going to end up paying taxes on something they may still take back :-(

This was in the state of NV, however I had already moved to CA by the time the additional automatic deposits came through.