There is no minimum requirement. As a courtesy to the other members and to get all the good from the organization as possible, you should participate in meetings regularly either as a speaker or in one of the other functions but you choose how often you can attend. Each speech is criticized. This is not to make you change but to help you build your speaking toolkit. The more tools you are experienced with the more options you have in various communication situations.
Toastmasters is very time consuming.
The amount of time you give to Toastmasters depends on you and your needs. You can also space your presentations to meet your schedule. Other program functions such as evaluator and timer require no preparation. You attend as many meetings as you can. It's just one lunch hour a week within a 10-15 minute trolley ride to the Conference Center.