How long do I have before I must enter Thailand after being approved for the LTR wealthy pensioner visa?

When you get approved, you will receive an endorsement letter. This letter is what you use to apply for your visa. The letter is only valid for 60 days.

You have 2 options for obtaining your visa, and you must choose only one, and you cannot change once you choose.

1) Apply to your local Thai embassy through the e-visa system. This will result in you paying more than the 50,000 baht that is the official price at most embassies, because they set their own exchange rate. It also means your visa will be a pdf document that you will need to print and carry around for 10 years. Most embassies will not process an application if you indicate your travel is greater than 3 months away. Since your endorsement letter is only valid for 60 days, you need to apply within that time and indicate that your travel will be within 3 months from the time you apply. Once you have the visa, there is no requirement to enter Thailand. The visa will be valid for 5 years from the day it is issued. You can enter any time within those 5 years, or not at all. The choice is up to you. Shortly before the 5 years ends you'll need to re-qualify for your 2nd 5 years. No one knows what this process entails yet.

2) Travel to Thailand and make an appointment with the BOI to pickup your visa in Bangkok. This will result in you paying exactly 50,000 baht directly to immigration, and you will receive both a 5 year and 10 year stamp in your passport, so there is nothing extra for you to carry around. If you won't be in Thailand within about 45 days of your endorsement letter being issued, then this option would not be available to you as the letter would expire before you could get the visa and you would have to apply all over again.