Actually there is no specific time to start, but as investors, of course we have special considerations when the best time to buy, and in my opinion the faster is better, this is because the opportunity for rising can occur at any time, the usual mistake is too often doing analysis and delay to buy.
But it is not good idea to invest all your money at the time of bull run. Price can fall down any time also and for this we have to invest portion by portion. Bear market is suitable to enter the market. Here anyone can get huge opportunity to buy more with low capital.