How long does it take to get the offer letter after verbal offer?

Depends on the company really

In my case Disney sent the written offer while I was on the phone with the recruiter who made the verbal offer. Other companies maybe a few days later.

I know the feeling, the constant checking of emails. Just remember as much as you want them, same way they need you as well. So chill out and you'll get your offer letter soon. Usually it takes around a week after verbal offer in most orgs.

I've gotten the letter before the verbal offer several times. So averaging it all together I'd say 0 time.