Both!
There are often limits on amount of product per year which is related to human and environmental safety. For commercial pesticides it is listed in quantity / acre / year.
Then there can be statements on the label regarding number of consecutive applications before switching to a pesticide with a different “Mode Of Action” (Group). This is where resistance comes in. For example, if one of your pesticides works by inhibiting respiration and another by neurotoxicity, usually a pest is toast because the successive generations can’t build a resistance to both at the same time.
Spinosad is fairly effective against weak pests, especially eggs. Some colonies of aphids have been known to develop a moderate resistance so I alternate.
Insects (e.g. White Fly) quickly develop resistance to Pyrethrin. It is ineffective here.