Hai there!
I was playing a server, on my shammy.
I noticed, I was drinking, and by misstake pressed my logout button, (I was inside an inn) When I logged in, I still had the drink buff, but when I moved, it disappeared, when I checked my mana regen it was over 500.
Yup, the effect stays if you relog while drinking.
The mana regen depends on how good the mana drink is, like a lvl 1 drink wouldn't give so much regen, but high enough for low levels.
Also, it regens while casting.
Here we go again, with steps:
1. Get mana drinks, mage water walks aswell.
2. Use it. NOTE: The duration of the drink MUST be longer than 22 Seconds.
3. Logout while drinking, takes 20 secs. Don't know if alt F4 works but anyway.
4. Login again.
5. Check your mana regen.
6. Enjoy
Prepare to never Mana break
Have fun!
Note: The server I play on is Trinity or MaNGOS modified.