everytime is the right time,but only if you invested in Bitcoin first before other currency.
altcoins are too many to choose but if you will stick in the top 5-10 then the safer your investment will be.try not to sneak for shitcoins because trust me the chance of profit is too little than to expect.
There are always some altcoins, which are profitable for investing in. But that is the question to find the best, as you above mentioned top altcoins are the best choice for making a profit in the future. But l think it can take a bit of time, l mean if you are a long term investor, you should prefer top altcoins, on the other hand, it is not a great idea.
Any long term investor will buy top altcoin as much as they can when it in a downtrend. But not every altcoin at the top is a good investment since there the one like YFI which only got to near the top thanks to the pump. Right time to buy some altcoins? Yes but spend your time on research which is the best altcoin that will have strongest price recovery to put your money in.