> For instance, let's say right now, the market is headed up. When is the right time to "re-balance" to more cash? This is just assuming that the market is going to go lower than it currently is, which is just as much a gamble as thinking it'll go up for the next year.
It's the other way round, if the market has gone up then you rebalance to hold more cash. You're betting on mean reversion, not just making a random directional bet; in the long term that works, and since you're not leveraging there's no "remain irrational longer than you remain solvent" problem.
> When is the right time to "re-balance" to more cash?
In theory if you wanted to invest "perfectly" you'd do it continuously. In practice trading costs, tax concerns, and the cost of your own time mean you want to set a schedule that's not too inconvenient.
> Talk to a financial advisor and choose a risk-based investment strategy that makes sense for your point in life.
I know this is the standard advice, but these days it's pretty outdated IMO. A financial advisor will rarely tell you anything more than the basic middle-of-the-road advice you find on the internet or elsewhere, and they'll charge you a substantial amount for the privilege.