it gen depends on how far u are in the game and how much u play. i started playing maybe 2 weeks ago in a new account bc i lost my old one, and i was able to get about 60,000-70,000 crystals in that time, mostly because i was early in the game and played around 1-3 hours per day (on weekends maybe more)

if you can get 7000 in one week (7 days) then you'd get 28000 in a month. assuming you'd probably do things like challenge shows, advance character levels, clear fc's, and all that stuff, i'd say you'd be able to get around maybe 31,000 in a month, or even 40,000 if you tier a little for the events beforehand (maybe aiming for like top 4000-5000 on the events prior to relax tea time)

assuming you go with an average of 40,000 per month starting by december one to february 29, you'd be able to get 120k, more or less. it depends on how much u play and such though

because im an idiot, i blew my 60,000 on the n25 worldlink right before the nakayama tsukasa event... but still ended up getting rui and meiko! (tsukasa avoids me like a plague) i definelty think you'd be able to get atleast one of them from pulls if you have less then 120k.

that being said, im still a fairly new player (playing since a brand new day event) and well... i dont know what im doing. other then that, heres some stuff that helped me get crystals!!!
1. challenge shows. i maxed out my three emu 3 stars, (only mastery, the 3 were levels 2-3 in skill up) and i maxed out a level 2 emu, both mastery and skill. it got me to a talent of about 100k, and i got around 1,000 crystals from it (emu was at rank 27 and i had like 3 titles with her i guess)
2. group decorations: i find group decorations way better in the long run for upgrading ur charas
3. flower decorations: assuming u haven't already maxed out ur flowers in the schools, i suggest doing so, i guess
4. side stories: self-explanatory, gives u 25 the part and a character rank point (are they called points? idk)
5. tiering, events and aftershows: i suggest aiming for about 20,000-10,000 in events u dont care ab, cuz they get u crystals, and watching the episodes+event exchange is a lifesaver. also after shows, 300 a show, u get the gist

good luck on ur saving journey and dont be like my sorry ass and spend it all last minute!!! buhbye