That's always a difficult question to ask. Based on my experience so far, I'd say it's better to cash out while up at the cost of winning more, than end up losing everything and wanting to have cashed out so damn bad. Had various experiences on Dplay and Stake during these past couple of months, but this is the general logic I came up with. If only it was that easy to abide by it whilst you were subjectively involved lol...
This is also my experienced before and since then, I'm cashing out everything that I can and I'd be more than happy to enjoy that winning.
There's always risk here and there's no assurance to have that luck until your bet ends, so do your best decision and make sure you don't regret it later on. That's already a huge money to deal with, your contentment will matter here and your goal of course.