The worst that could happen is that you try to enforce the new rule and the validity of the rule is challenged and it turns out to have some flaw.
My own caution would be if there are particularly belligerent or litigious residents in the block. Unless your by-law is watertight then you could be tied up for months with challenges and appeals which is going to cost you time, energy and money anyway. A good lawyer will already know all the potential landmines and tripwires.
For instance, what if you define the maximum visitor period as, say two hours, and a rogue parker just moves to another spot or takes the car for a spin round the car park every couple of hours.
You wouldn’t need a by-law unless there was some selfish person already bending the rules. Don’t sink your ship for a ha’pence worth of tar.