Clock without RTC. Drift?

If u construct an clock with a square wave, how much will it drift? And is it even possible?
In my head its possible with some count nodes etc? Since they can hold values up to 255? So that would be enough for hr, mins and sec since u reset count at 60 and 24?

But how much will it drift? Is it because of how much load the “cpu” has? Maybe u even could compensate the drift easily?