It's less than two months and at higher level can do it even in a month. The islands are for 50+ so there are daily shop mini quests for shards and later the Pathfinder crystals. Weekly 1d prem doubles the daily crystals. BB has the game analytics about everything so they probably decided this based on all the info there is. A lot of people have gotten their gem pits just by spamming short letters. I'ts up to the player to decide where to use the crystals.
I also forgot the gifted gems or event gems which Alex mentioned here: https://forum.tsotesting.com/threads...ll=1#post49162
Previous Crisis was over 1k gems at least for me IIRC.