Different types of gear will have different payback periods, and different longevity.
Buying off-camera lighting might last you 4 to 8 years. But you may not use them on every shoot.
Buying a good quality lens will last you 5 to 10 years. Depending on the lens, these will probably get the most use.
Buying a good quality body will last you 2 to 3 years. You can't take a photo without a body, but whether this is your primary body or your secondary / backup will matter.
In each scenario, I would approach the purchase differently. By all means, don't put yourself into debt without having a plan to recoup the money. If you have one body and are committing to shoot a wedding, then you absolutely need to have a second body to cover your ass and protect the potential income. If you already have a short, medium, and long lens, and you want some sort of niche or artsy style lens because there's one shot that you want to try and get with that lens, then absolutely hold off on the purchase until you're in a better financial state.
When I bought my first DSLR, I got it at Best Buy and was given a 1 year same-as-cash financing plan. I paid $100 per month, every month, deducted from my freelance earnings, and paid it off on time. Some months I made $300, other months I made $1,500. But I made sure that the first $100 I made went straight to the camera. Other revenue went to some extra lenses, a flash, and some other accessories. 1 year later, I had a fully paid off kit, and continued to grow my revenue stream. I went into debt with a specific plan, stuck to that plan, and have been set ever since.
More than 10 years later, I am still earning good side money from photography and freelance work. It's not my main gig, but it's enough to report to the tax guy.