You will see the odd person at the roulette table keeping a record of what numbers come out. Nowadays, the casino will show the last 15 or so numbers plus the percentage of blacks and reds. I guess this is to appeal to the people that believe in gamblers fallacy in that after a series of reds, there has to be due a black.
Whereas keeping track of the numbers on the roulette, won’t help you win, keeping track of your wins and losses will help you decide when to stop. The level of detail is up to you. Its probably not necessary to record each bet but the win/loss of each session should be noted, especially if you are playing at different places. The time and duration of play should be noted as well as any systems used.
Online, if you are playing at the same place, this is easier as you just have to keep track of the deposits and withdrawals. This can be easily done in a spreadsheet program such as Excel or Google Docs. The advantage of this is that you can quickly add up your winning or losses.
You may think keeping a mental note is fine but as gambling is so emotional, its sometimes difficult to keep track of the exact gain/ loss. You can feel vastly different depending on the order of wins and losses. For example, if you won £100 straight off, you feel better than if you won £200 and then lost £100. The net result is the same but due to the way the brain works the latter is worse. Using a spreadsheet will allow you to think objectively about the results.
Once you have have your statistics in front of you in a spreadsheet, its easy to see the total wins/ losses but also the time that you play. You may realise you play a lot more than you thought or lose more than you thought.