Assuming that, nowadays, BTC's hodl could be as long as 5 or even 10 years from now.
How long is a long time hodl in alt-coins?
For example, if I'm looking to hodl ARK/STRAT/XMR, what is the right time-frame? is it just for a few months to a 1 year, or can I leave them for 5 years with the hope of weathering the storms and coming out strong?
And besides, do these kind of coins have a 5-10 year lifespan?
Nothing more than a year. I don't think you want to leave your stash idle for periods longer than that.
I would be especially nervous about the current rally. You never know when the market will turn.