The stock levels return one step towards initial levels every minute. A step is one seed, unless stock is more than 200 greater than the initial stock, then it is 1% of the overstock.
The rarity of each seed is based roughly on its level requirement. It takes roughly 10 seconds before you can steal again.
Interestingly, all vegetable, flower and hop seeds can be stolen from the stalls, but not all of them can be bought directly from her. She will also buy the seeds, but only if the player has not stolen from her stalls, from Martin the Master Gardener or from a master farmer within the last 30 minutes. It is easier to steal seeds if you trap Martin in the pig pen just north of the market.