Temperature question on flowering

Hi there , with lights on im getting about 21.5 / 22 º C (70.7/71.6 F ) but its getting colder each weak , when should i start worrying about getting an heater ?

Soon. Get one of those oil filled radiator heaters that has a temp setting. No fire risk and more efficient.

If you don't already have one, look into a fan controller.

Will save you fortunes on heating costs. Do watch humidity after week 5 or so of flower though, better to run extra heating and keep the humidity down and air flow up for bud rot issues.

minimun 18º or 20ºc?