There are a number of ways, the most readable being 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.

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.

There are a number of ways, the most readable being 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

There are a number of ways, the most readable being 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.