i base a lot of my trades on fibs, and previous highs, fractals,... sell a small portion on every target hit.. and eventually you can sell at the top. (mostly with alts - you can also do this vice versa to find bottoms).

in crypto it doesn't make sense to sell all your assets, otherwise you'll find yourself gruding why you didn't hold longer or sell sooner.. If you bought a lot of bottoms in December, then the best strategy is to sell 10%, 25% or 50% of your assets when they all hit between 150-500% increases, and hodl the rest.. truth is we don't really know where any of these coins are going (we can guess and estimate), but most people who sold at the top is based on luck...

when all indicators show oversold, out of bollingers, exhausted, then by all means make a small sell.. i just wouldn't sell your whole position. Save those earnings as ammo when high volatility hits again, and big dumps come in play..

hope that helps a little, in essence there is never really a "right time to sell", should be up to you, and if you are pleased with a x2 gain,  by all means congratulate yourself and be happy with your earnings!!!  

happy trading/investing!