I think the right time to borrow bitcoin is when you play a trade, and get the information that there is a coin that cost would be very high. maybe it can make us a profit
If you are sure about some investments that can yield genuine profits only then you should borrow the bitcoins and invest. On the other hand if you borrow just to play gambling then if you lost in gamble then how will you return the borrowed amount to the lender ? Lender should also make sure he is giving to a person whom is really needy and he has some surety that his lended money should be returned on time.