I think that you can start now. But do it carefully. My view is that bitcoin will continue falling to 2900. But after it, the new bullrun will start and it will be more faster and bigger.
I would say that 1/2 that, or about $1400, I have long felt BTC would go back to around $1k, where it was begin of 2017;
It's more important that is 'mean-revert' and stay in that window, this 'moon-shot' crap is over,
BTC went to the moon ( $20k) because the kids were buying it on credit cards ( that's over, banks closed that window )
BTC went to moon because FED was printing free-fiat 2008->2017, that window is now closed
So now BTC will revert to the mean, and that means pump around $1k,
I have always had the opinion that BTC should be MINED and NOT bought, but honestly the point of inflection has already passed, its now more profitable, to harvest 'lost' btc than to mine, this is largely because the antminer s-9 class machine are making a whopping -1$ a day, that's right, everyday you run an s-9 you lose a dollar. When BTC goes to $1,000 or less, then it will be what -$5/day
There are three ways to get btc,
1.) buy it ( the dumbest idea )
2.) mine it ( no longer profitable ), think this way it cost more to mine, than to buy it,
3.) write your own software to harvest, scan, or search for 'lost' btc, this is a good use of re-deploying those GPU racks
www.inflection.topWhen is the best time to INVEST?
Well when your young, and you can afford to lose all your money. The problem is BTC now has a 90% volatility, its just as likley to go to $300 tomorrow, as it is to go back to the $6500 zone, but for now the trend is DOWN, because of what I said above with the closing of the FED FIAT, and the credit-card, there is no new money coming to BTC, which means now it slides down to its TRUE VALUE.
If there is a WAR anytime soon, in EU/USA, ... then I would expect BTC to skyrocket, as its a safer way to move money than GOLD. Other than that there is no way BTC will do anything other than bounce around and slowly revert to its mean-value.