Although I've been known to keep trying combinations just for a reason to keep shooting
I'll normally do the following.
1) Find some reputable recommendations for a powder/primer combo.
2) Decide what I want from the load velocity wise.
3) Check Quickload and my books for the range of powder expected to reach the velocity I'm wanting to achieve.
4) Load 10-30 rounds with 3 rounds/ powder level (for example if my range was 41-45gr's, I'd load 3 at 41, 41.5, 42, 42.5, 43, 43.5, 44, 44.5 and 45 for a total of 27 rounds.
5) I'd shoot at 9 different spots on a target and shoot each load over a chronograph.
6) Based on the velocity readings and the single 3 shot groups, I'd load up 10 or more rounds at the selected load and shoot them in groups of 5 over the chrono.
7) Assuming they shoot pretty well, I'd load some up and leave them long. Then experiment with 5 shot groups at different seating depths.
By this time I normally have a load that is shooting in the velocity range I was looking for, and is shooting up to the accuracy I expect from that particular rifle.
To answer your original question, I don't stop until I have a load that shoots up to my expectation for that rifle. Sometimes, its the first load I try. Sometimes it takes multiple powders, primers and bullets. I tried hard to make Nosler Ballistic tips work in my 7mm RM and could never get below about 1MOA. I switched to A-Max's and started shooting sub 1/2moa immediately, with all other components the same!
I'm normally not happy until 1/2moa groups are common, much better than that and I'll need someone else to shoot the rifle for me.