Hello andreaww
There is no restriction, neither in the number of depositions (provided you respect the minimum duration) nor in the point of time the first deposition is made.
However, I wondered why you wanted to sign up for an annual GA while you are unsure whether you will like / need it or not?
Initially, you could opt for the 1-month GA travel card that is valid for one month, it is a full GA travel card, yet does not come with any strings attached such as a minimum contract period.
https://www.sbb.ch/en/tickets-offers/travelcards/ga-travelcard/ga-1-month.html
Or, what about opting for a Half Fare Card and EasyRide. By doing so, you would have all the benefits of a GA (unlimited travel on the GA network, hop on/off when and wherever you want), you could test the liberty of unlimited travel, while not being forced to any prepayments or contracts.
Kind regards,
Roland