When will Bitcoin drop to $500 again?

When will Bitcoin drop to $500 again?
When everybody forgets about it - including you.

I want to invest on bitcoins but the rate is around $1650. I'm waiting for it to drop to $500 so I can purchase a good number and sell it off later when it crosses back to $1000.
If the only reason why you want to see a price of $500 is to buy and sell again at $1000, the price is irrelevant. What you are looking for is an investment to double your money.

This investment could still be Bitcoin. For example, if you had bought at $1650, today you’d be up 33% or 1/3 of your 2x return goal already ($2200).

However, investments are never risk-free. Nobody can guarantee that the price will keep rising. Similarly, if the price - for some reason - falls to $500, it may never reach $1000 again.