How many months can somebody hang on to a check before that check is void?

My cousin was given a baby shower 2.5 months ago. I couldn't attend so I sent her a card with a 40 dollar check inside. She wrote me a thank you letter, but now I see that she still has not cashed the check. How many months can she hold that check without cashing it before it becomes null and void? Is it 3... show more My cousin was given a baby shower 2.5 months ago. I couldn't attend so I sent her a card with a 40 dollar check inside. She wrote me a thank you letter, but now I see that she still has not cashed the check. How many months can she hold that check without cashing it before it becomes null and void? Is it 3 months or 6 months? I need to know so I can properly balance my check book!
Update: Oh, I know she is really busy. I didn't want to bug her is she had 6 months time, but if it's 3 months, I need to get on her about this. I know she could use that money too.
Update 2: dmg- I am not stupid, I deducted the amount and everything, it's just that I have to go back all those months to figure out why my statement and my register are always 40 dollars off and it is very annoying.
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