How do you know when to cash out??
When you need the money. Unless you are in a hurry to cash out, there is no reason to cash out right now. Wait until something like December happens before you even consider cashing out. If something like that happens once again you can cash out some but do not spend the money on silly things like I once did. It is a wrong move, just put it aside and wait until all coins go back down on a bear market and sees the dip, than buy again but higher amounts this time.Now is probably not the best time to cash out. I'd say wait until it reaches at least 20k or higher and then you can start thinking about it.
When we believe into bitcoin to reach $40k or $60k then how $20k will be the best time to cash out ? I suggest not rely on bitcoin prices for cashing out. Just calculate your ROI and if you are satisfied with that then you may go for it, else wait more 
. Everyone might have different expectation on bitcoin price levels but definitely not with ROI and waiting periods as these are personal things.