Hallry your question is very open ended. We've been involved in FTC two years, made it to World last year. Your goals will dictate the hours needed to obtain your goal plus as someone else mentioned, its also tied to the competitions you choose to attend.
For my team, we determined we didn't meet enough in our first year and in our 2nd FTC year, we averaged about 10 hrs. of mtg time per week from Sept kick off - all the way to World Tournament in April ( season length is another discussion ). Unless you really nail it at the first competition you attend, your bot will undergo some level of evolution between competitions ( sometimes even a revolution ).
Team size can help lower the amount of hours per team member. If you have a large team - say 10, you could have 2-3 students per night working on robot with no trouble and the kids rotate nights. You will need a solid design plan so the design doesn't evolve between nights.
