I could give a few more examples (mirrors edge glass sliding, pipe sliding) but there isn’t a X amount of seconds per sound type. It depends on what is recorded. That tomb raider sample I sent is I think < 1 second, and this fits the game in its time, but might not fit yours. There will also be loops that are longer (rain, wind) in ambient music and games in general that can’t possibly fit within that time properly.
The best advise I can give if you look for a sound effect online, is that the provider of that effect allows you to preview (download) it so you can test it before buying a license for use. Even better if the provider sells looped sounds. If a sound ends different from how it starts, you might just have to cut half of it to hopefully get remotely close to creating a looping sound the hard way.
I’d like to think from the end user’s perspective. The sound is going to loop, that’s expected. It doesn’t have to be 20 seconds long. But most importantly, it should not be annoying to listen to, fit the overall sound design of your project, and fit the situation. Sounds can also be dynamically modified at runtime (pitch, volume etc.) within Unreal Engine to provide dynamics. An end user will not enjoy a sound of sliding on stone when the visual design is walking on mud. The balance is critical. Again, it’s most important to be allowed to test the sounds and have a library to pick from before spending money and time on it.
I’d definitely not go buying short (< 2 seconds) loose files without having selected them with a purpose. You’d be better off using samples cut from real recordings on youtube to test your sound design. You’d better be off buying a cheap mic to record them with materials found nearby.
A lot of sound artists actually create sound effects in a mic using materials you would not expect, but sound natural. It’s best to keep in mind that everything is designed with a goal in mind. You can find two packs on the internet that sound good initially but still sound wrong / weird when mixed and put in your project. That’s because they use different materials, techniques, hardware, mixing, quality with different goals (styles) in mind.
Testing this is crucial and I hope you find a way (perhaps a full sound pack, or by sampling youtube) that you can get a better idea of the whole.
If you are comfortable with command line, you can use an open source video downloader for samples to test with like:
Github: YTDL
I’d be very careful with the overall sound effect seller. Copyrighted and AI generated content is sold too often.
If you look for a mic, you can honestly often get away with the cheapest thing available. I use a somewhat expensive Blue Yeti mic which is very portable to take with you outside (usb connector, pop / fluff filter etc.) it’s tiny, clear and popular in the ASMR community as well.