There are a number of ways.  Assuming this is about typing rather than handwriting: The most readable way is to use the Unicode character for one-half:

> 3½ hours.

If you don't have easy access to the Unicode character set, then it's common to put a space between the whole numbers and the fractional numbers:

> 3 1/2 hours.

Naturally you can also use decimal numbers, but this sounds more scientific or technical:

> 3.5 hours

I expect these are common to many languages (not just English), and I'm not sure how it would be different in another language with a different primary alphabet.