Up to a day before date the visa expires. No other limit. However of you don't enter within the year the the visa expires and you have to start the visa process again. I entered 4 months after visa start date without question.
I'm in Thailand on original 1 year visa O-A retirement out of USA. I understand I'll have to apply for extension of stay after I reach the end of the admit to date. I plan on having the 800K baht in a thai bank account a minimum of 2 months prior to applying for extension of stay. Not sure I understand how the requirement to have funds for 3 months after the application date is proved since the extension will be my first.
But I will say it can't hurt to have it included. Just ask for a criminal background check for immigration purposes at the live Scan location. The process is simple.