2 times longer

I'd say "Student B studies twice as long as Student A".

Or

"Student B studies twice as much as Student A".

Yeah, so would I but my student wanted to know how to express that using 'than'. anyway it turned out to be a trap 😛 because as soon as I said it she said 'Doesn't that mean Student B studies 9 hours?'

I also tried to use '3 hours more than Student A' after the fact but she wouldn't let my first attempt go. So I thought I'd ask some others.

All of them are used. Some examples from Google :

• A rectangular football practice field is 2 times as long as it is wide...
• Servants at stately homes lived almost twice as long as other Victorian workers

• This exam is two times longer than the real one could ever be
• VS 2005 SP1 redistributable takes twice longer to install

• Backups taking 2 to 3 times longer than usual.
• Group 233 was switched from twice to three times daily milking at 100 d ...

I'd say twice as long \ much.
I've always avoided saying 'two times' - it may be a British \ American thing but I'm not sure.

I wondered that too - but a lot of the examples I found on Google were from UK websites.

So, do you think "Student B studies 2 times longer than Student A." means student B studies 6 hours or 9 hours?

I'd say 6 but if I had to nail down the grammar mathematically I can't argue that it's not 9... other than I think it means 6.

Not being mathematically minded it probably took me two times longer than anyone else to work out how it could possibly mean 9 🙂 I finally got there - but I think anyone using it spontaneously would mean 6.