5-6 hours a night is far far too much for most people as you are most likely playing until your arms are dropping off and/or you are bored with the session. Neither of these help your game in the long run.
If you are not a pro then around an hour or so, maybe up to 2 hours, should be more than enough practice. Make sure you are doing proper practice as well - don't just throw for the 20's to see how many 180's you can get, work on all aspects of the game such as scoring, finishing, setting up finishes, etc. Even going round the board on the big sections of the numbers is good as you quite often need to hit a big number to get on a double - eg 43 left you need either a single 3 to leave tops or an 11 to leave 32.
Also, if you do go for a couple of hours practice per night have a little break of 10 to 20 minutes in the middle.