Answer: Rosemary grows slowly compared to many other herbs. It is a shrubby type of plant and can be pruned as needed to keep it in bounds (or to use it in the kitchen). Since its main requirement is good drainage, it is best not to repot it until it shows signs of truly needing either a larger pot or refreshed soil mix. Indications for repotting would include roots growing out the drainage hole, need for frequent watering, or general lack of vigor. Another sign would be when the soil has become quite compacted over time and dropped well below the rim of the pot.
Basil on the other hand grows relatively quickly and allows you successive harvests. Depending on the variety you are growing you might need up to a six inch pot. (The signs of needing repotting would be the same as above.) Eventually the plant will exhaust itself and go into decline. At that point you will probably prefer to replace it and that would be the time to replace the soil also.
When you do repot, avoid the temptation to jump to a much bigger size immediately. Instead, go up by only a half inch on small pots, and perhaps an inch on larger pots. This progression allows you to refresh the soil periodically and also reduces the risk of inadvertently overwatering.
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