You could set a short period of time but make it clear from the language that he hast to blow clean each time. so, for example, make it six months, but worded that after six months of zero positive readings on the breathalyzer, the requirement terminates, rather than saying he will submit to a breathalyzer for the next six months.

as someone with a testing requirement in my custody agreement, six months is a pretty long time not to screw it up from their perspective.

my order is open ended on the testing, but there are changes to the custody agreement after six months “of sobriety“. So from my perspective, when I know he has been drinking, that resets the six-month clock every time.