It is a subjective thing and each person will have their own way of deciding when this is a good idea or not, there are many that just buy bitcoin whenever they have some cash available, others use DCA, others prefer to use some technical analysis to decide when it is a good time to invest in bitcoin, while others just use their instincts to do so, now if you are planning to hold your coins for the long term almost all methods will produce profits, but if you want to trade then you will need to use TA almost exclusively to take the decision of when to buy.