Basically when they are getting ready to settle down for winter. Different plants, different climates, different times.

plants in pots about 6 weeks before I expect them to stop flowering. The ones in the ground rarely get fed unless they are showing signs of needing it.

There is a rule of thumb for fertilising - for permanent plants in the ground, stop fertilising when the longest day has passed at the very latest; you don't want to encourage more soft growth which won't have time to harden off before autumn/winter. I personally don't fertilise those frequently anyway, other than raking in some Growmore round about April and then a mulch with composted materials. For temporary, flowering plants, continue to fertilise as long as the plants are viable - they're gonna be dying anyway as soon as colder weather arrives, so no point in worrying about hardening growth.