PADI eLearning is valid for 12 months from the first date you log into the PADI eLearning website.
So you need to plan on traveling to Koh Tao to complete the PADI Open Water course within 12 months of commencing PADI eLearning.
When you do travel to Thailand for the open water dives, you want the scuba diving theory to be fresh in your memory. So if you completed PADI eLearning several months ahead of traveling to Koh Tao, it’s worth rereading the content to refresh your diving knowledge.