He’ll just flat out lose them, or forget about them, or put off depositing them for whatever “reason” (one of the things we’re working on right now). You’d think that depositing a check would be simple and good news and happy and something to look forward to, but there’s a whole host of situations where folks won’t want to cash them. I’m not saying it’s logical or that it makes sense, but it happens.
Generally speaking, most checks go void at the 180 day point if no other limitation is placed on them – banks simply won’t honor them after that point. Some banks and institutions have watermarks which specifically say that any given check is null and void in less time. An employer I worked for awhile back had a watermark on their paychecks which said they were only valid for 90 days.