I started my first webcomic in 2008. The next year I stuck some small Project Wonderful ads on my site (though I mostly used those pennies to buy space on other PW ads), and a couple of times I put together some stapled books to sell at a convention I was going to be helping out at, but nothing really serious. I finally put together an actual book to sell in 2010, along with a couple of prints. So I guess it was about a year and half, two years? before I started really trying to see if I could make and sell stuff from my first comic.
I'd say it also depends on what you mean by "monetise." If you want to, say, turn on ad rev for your comic here on Tapastic, go for it! That doesn't really require anything of readers. Things like starting a patreon campaign don't involve any risk, but you'll get more of a response once you have readers who are really invested in you and your work, and that can take time to build up. If you want to sell products at cons, you have to have enough stuff to have a significant presence at your table, and enough content to make a product that's worth buying. If you want to sell products online, you gotta know that only a fraction of your readers will respond unless you hit on some super relatable merchandise (I've heard 1/100 as a standard for that fraction, and it doesn't sound super far off to me).
I don't think there's a specific amount of time or size of audience that signals you're ready to "monetise" -- it's gonna be different for every comic -- so I tend to kinda look at my readership and how they're responding to stuff to see if I might have people following my work who might be interested in buying things.