That's something only you can answer. Different people have different expectations.I put a 30lb dumbbell on it in the middle and got .005 inches of flex. I get about .001 inches when moving my z axis across it with no spindle on it so the question is what is too much flex and what should I shoot for?
The bigger issue is that it's going to twist when that same 30lb load is pushing on it from the end of the Z axis.
And the sag or twist isn't the only issue. The fact that it can move, means it can vibrate as well, causing poor quality cuts.