To make things easy to track, try to grind this out on one table so you can track how many hours you have easier. Once you have selected a table, load it up and don't touch anything. Your ball will sit at the starting spot (waiting to be shot off). Here, your ball is safe and you can now idle 100 hours. Personally I did this every other night, but the more you idle the sooner it will unlock.
Once you are ready to check how many hours you're at, shoot the ball off and fail until you get a game over. Now start up a new game, but this time select the pause button and quit out to the main menu. The reason for this is simple.
1. You're hours only get tracked once your game ends. This means you will have to end the game for the achievement to pop also.
2. You can only track your stats/time played on a certain table and only while you have a game active on that table.
So, if you have an active game and you're at the main menu, you will be able to check the stats for that table. Select Help & Options in the top right, and then Statistics. Your time played (for that table only) will be shown. This is why I suggest doing all the idling on 1 table, to make it easier to track.
There's nothing really to it. You just have to leave the game on a lot until you reach 100 hours. Which is 4 days 04 hours 00 minutes. Remember, the achievement doesn't unlock until you end the game.