First thing you have to establish is how good really is good enough? Is the rifle straight from the factory or a custom job? Will it be for high volume target/varmint shooting, long range targets and low round varmints, big game, F-class, bench rest, etc. ?
When I first got into reloading I didn't care how well my loads shot, I wanted to play with different powders, bullets, primers, and any other variable I could to see how many loads I could get to shoot well. I found my best load early on but it didn't stop me from constantly trying to find a new one. In the long run it was a waste of barrel life but the lessons I learned were invaluable and it gave me a ton of trigger time.
When I picked up my Browing X Bolt 300 WSM
I decided I would stop looking for loads if I could consistently get 5 shots under 3/4 MOA. The second load I tried shoots a very consistent 1/2 MOA for 3 shots and keeps 5 under 3/4. I quit doing load development and spent more time practicing in the field instead of at the range.
My 22-250 is for prairie dogs and feathered vermin and I wanted a load that would keep 5 shots under 1/2 MOA consistently. I found that load on my first day at the range and it's probably the only load that gun will ever shoot again unless component availability forces me to try something else.
My 6.5 creedmoor is my only custom rifle
and I wanted it to be a consistent 1/4 MOA rifle. 5 trips to the range yielded every load between 1/2 and 3/4 at 100 yards. I was frustrated that I couldn't get the loads as precise as I wanted them but then I realized I had a gun that was shooing 10 different loads under 3/4 why the heck was I complaining? A little tweaking with the best load produced sub 1/2 MOA 5 shot groups at 100 and has been boringly consistent at longer ranges in the field.
I guess my "standard" for a factory rifle would be 3/4 but I'd probably try and tweak it to get 1/2. A custom gun should definitely hold 1/2 MOA and I've never owned a bench rest rifle but I imagine 1/4 MOA would be a good starting point. However, some times playing with different loads is the funnest part and I will probably always try something different just to keep things interesting.