To answer your question, you can go as long as you want with a leaking HG. It won't suddenly explode and leave you stranded, if that's what you're asking. You could keep going.
Now, what should you do, is up to you. HG leaks can be from the coolant into the cylinder, from the oil into the cylinder, the coolant to the outside of the engine block, the oil to the outside, oil into the coolant, or any combination of the above. Compressed gasses in the cylinder can escape into the engine block or bubble through the coolant. This in turn, can foul the cat converter and emissions sensors, make a mess everywhere you park, taint the oil and cause various mayhem in the engine block, or any combination of the above. Loss of too much coolant or oil can lead to overheating or seizing, which can cause a catastrophic fail on the road. Or, it could just be an annoying leak for a year or so.
If you're keeping the car, start a saving fund for HG replacement, if not, the buyer should be made aware of the problem, to be fair. It's not an impossible DIY job and costs a lot less than a shop, but it is an advanced, protracted, repair. It's one of those types of jobs that the parts are a few hundred bucks, and the labor is a few thousand.