From the start, the game force me to spend 20k/week for record room or practice room (Not enough space). So I usually just hire only for record room or practice room. Should I spend for break room from the beginning too ? And when should I use hiatus for the girls ?
Take a look at the task screen to see the requirements for Leveling the different actions.
makeing 3 singles and holding a concert are part of that.
just compare the requirements for the next level of "performance" and keep doing it... also Jobs found by your assistant won't compare in regards to payout and stamina required in the long run... this will change if you have more idols as you can simply switch in between them.
If the girls don't train or do jobs the stamina is not going down much, once you earn 10k with a single action of "performance" you'll be good most of the time.
Also use digital release only for your releases dureing the 1st year, as this will reduce production costs and increase your pay out... once you have around 5-10k fans you can change to different release forms, aslong as you have enough money to produce them.
I think 150% of singles in comparison to fans is a good number.