No they don't and you can hold it as along as you want, but you risk losing it if your account gets banned.
I faced an issue sometime back with adsense wherein my balance was US$ 85 one day and next day it went to zero. I checked with Adsense and they did not respond. so, common sense suggests not to hold any adsense payment. Get the same and hold it in your bank if you do not want to spend