Of the countless tech-related questions I get from friends and family, one towers above all others: “When is the next iPhone coming out?” My answer is always the same: “I don’t know. But I have a pretty good guess!”

At the heart of this question is a fear shared by all would-be gadget buyers: that something better is just around the corner, so buying now would be foolish. But if not now, when?

There is a small but fertile online industry dedicated to answering precisely this question, and the sites that algorithmically answer your when-to-buy queries work quite well. Retrevo (retrevo.com) monitors activity at online retailers, news wires and review sites to compile a dossier on virtually every gadget in the world, including comparative price information, reviews and – crucially – a chart showing how far along a gadget is in its life cycle: “new”, “reaching its prime”, “over the hill” or “ready to retire”.

Decide (decide.com), a newer producttracking engine, gives more forceful advice, issuing a “buy” or “wait” recommendation and laying out products’ full release histories. (At the time of writing, Decide indexes only laptops, televisions and cameras; the company says additional categories will be added soon.)

Anyone with questions specifically about Apple’s products can refer to the venerable MacRumors Buyer’s Guide (buyersguide.mac rumors.com), which has been issuing clear and dependable when-to-buy advice for Apple gear since February 2003. “Some people enjoy reading the rumours,” says founder Arn Kim, “but the biggest reason to visit a site like ours is to get help making a buying decision.”

If these sites don’t settle your nerves, you can fall back on a few rules of thumb. Smartphones, tablets, cameras and televisions – most electronics, actually – are typically refreshed or replaced on a yearly schedule. Check the manufacturer’s website to see if your desired gadget has been on the market for more than nine months, and adjust your plans accordingly.

Laptops and desktop PCs are trickier. Intel, which makes the processors in most of the computers on the market today, follows what it calls a tick-tock upgrade cycle. Every other year the company revamps the way it makes its chips so that its factories can squeeze more processors out of a given quantity of silicon (this is known as a die shrink, and results in lower prices and leaps in processor efficiency); the last time this happened was early 2010. In between these updates are smaller changes to the chips, called microarchitecture updates.

Buying at the beginning of the “tick” – the die-shrink phase – practically guarantees against buyer’s remorse, but waiting a full year is often not an option. Computers with processors younger than nine months are a safe bet.

Lastly, beware of the sale trap. Unusually low prices may be a sign that a retailer knows something you don’t and needs to clear its inventory to make way for an imminent replacement. Nothing ruins a great deal like instant obsolescence.