Here are a few considerations if you need a backup generator to run for days on end.

For the sake of simplicity, there are two fundamental types of generators: Inverter and non-inverter.

Inverter generators are relatively new and offers several advantages compared to non-inverter generators. But first, you need to understand the differences:

1. Non-inverter generator - The engine turns an AC generator. The output powers the load directly. As such, the engine needs to run at a constant 3,600 RPM to maintain the 60Hz frequency. The output voltage is often regulated using an AVR to maintain the required output voltage (220-240V).

Pros: Cheaper upfront cost and maintenance. Can often be field-repaired due to their simplicity. Perfect for short-duration backup use
Cons: Running at a constant 3,600 RPM makes it noisy (open frame ones), has bad fuel economy, and depending on the quality of the generator windings, it also often produces a more dirty AC output (jagged sine waves).

2. Inverter generator - The engine turns a 3-phase AC generator. The output is converted into high-voltage DC then inverted back to AC, which then powers the load. The engine speed varies depending on the load demand.... ie. it runs at about 2,800 RPM when there's little to no load but them ramps up to 3,800-4,300 RPM when on full-load.

Pros: More fuel-efficient. Produces cleaner AC waveform output (pure sine wave), parallel capability (you can combine the power of two similar inverter generators), and they often run quieter when not at full-load (lower RPM and they're often in an enclosure)
Cons: Higher upfront cost and can be expensive to repair. Parts like the inverter module can also be prohibitively expensive to replace

Non-inverter generators are the most common type. These can be broken down into different categories... portable backup, continuous duty, prime duty, etc.

Portable or small non-inverter backup generators are meant to be used sporadically during temporary outages. They are made to a cost and have a relatively short lifespan, owing to the fact that they're mostly air-cooled uses an oil-splash lubrication system. You can force them run them continuously (stopping only for refueling and maintenance), but since their designed lifespan isn't that long, you won't get much use of them after a while. Note that these run at a nominal 3600 RPM all the time to maintain the 60Hz AC power output. Typical lifespan is just in the hundreds of hours.

Then there's the commercial-grade generators that can run pretty much continuously for days. These will often have water-cooling and a pressurized oil system for better reliability. Some have twice as many poles (4-poles) in the generator that allows the engine to run at half speed as the previous one, or 1800 RPM. Lifespan is in the thousands, even tens of thousands of hours. You see these being used in hospitals, malls, commercial buildings, etc.

On runtime, I wouldn't recommend that you keep it on for 24 hours, unless you really have to. Again, their lifespan is finite. You're technically running down the hours of their life by using them. In my use case, we typically need the generator running at 12-16 hours a day for every 24 hours the power is out. It's mostly there to run the A/C and other creature comforts. But this has not happened, so far. My 3 year old non-inverter generator only has about 14 or 15 hours of total engine hours. At this rate, and if I keep the maintenance regimen up, I can probably make it last decades.