I assume you understand the length of standard notes and that your question is about the length of the “tuplets”, that is, the ones with a number.
Sadly, the system is not fully consistent, but we come to the weird cases in a moment.
The number in a tuplet is shorthand for a division. That is “3” is mostly short for “3:2”, meaning “three notes in the time of two of the standard length”. In the same way “5” is mostly short for “5:4”. Sometimes it is written like this to avoid confusion.
In 4/4 and 2/2 time signatures, everything is divided in powers of two (2,4,8,16,…) and in these times a tuplet would always fit in the previous power of two:
- 3 is short for 3:2
- 5 is short for 5:4
- 7 is short for 7:4
- …
- 15 is short for 15:8
- …
Well… not always, for higher numbers, some music notators and musicians (like Esther in the other answer) feel for example that 15 should be short for 15:16. So, you might have to look at the context to figure out what is meant.
Another complexity comes with other time signatures: take 6/8 for example. Its bar is divided into 2 dotted 4th notes, then in 6 eight notes, then in 12 sixteenths etc… and so a quintuplet of 5 eight notes is to be played in the duration of 3 eights. Hence here “5” is short for “5:3”
Here is the rule for tuplets with just one number:
Tuplets are usually to be played faster than if the number wasn’t there and should fit equally in a standard time unit, as defined by the time signature.
Common exceptions where tuplets are played slower:
- duplets “2” is usually shorthand for 2:3 (eg. Duplet eights in 6/8 time signature)
- 4 can be shorthand for 4:5 eg. in a 4/5 time signature.
Source: experience & Wikipedia
https://en.wikipedia.org/wiki/Tuplet