In every band I was ever in there was certain bar its members understood they were expected to play up to. And in the better bands that bar was constantly being raised.
Members who weren’t up to the standard , or were unwilling or unable to up their game to meet that standard, were eventually let go after appropriate warnings or attempts at remediation failed.
So I guess you could say every member of the bands I was in was on perpetual probation. No one was exempt, or could consider themselves ‘safe’ if they weren't cutting it. And in every case whether or not you were cutting it was pretty obvious to both yourself and everyone else in the band.
Now in fairness, these bands all had ambitions and fairly serious goals. So the music and money far outweighed the hang as far as their members were concerned. With a more casual “night out with the guys” social circle type band that might be too extreme an approach to take. But a little intolerance for slacking off never hurt any band IMO. Because what’s the point if the results aren’t worth listening to because one or more bandmates aren’t carrying their fair share of the load?
So yeah, probation is a good idea IMO. But I wouldn’t lock myself into a fixed timeframe about it. Longtime members are just as capable of slacking off, developing personal issues, or just losing interest as a newbie member is. So be upfront about goals and expectations and let each bandmate show by their actions whether they intend to remain in the band or not.
FWIW, most times when it did become necessary to cut someone loose, the guy himself walked voluntarily well before it reached the point where the band was forced to give him the axe. Actual firings were extremely rare. And except for a few times, all the departures were cordial and politely handled because the band’s expectations were clearly understood and shared.
Ok. End of sermon. Good luck!