Your visa application should show onward travel within 60 days.
Your airline is likely to check this as well.
If you get a 60 day TR visa, do print it out, you'll need it when first arriving in LOS and you'll also need to show it to obtain your 30 day extension.
One 30 day extension is allowed with the 60 day visa, it costs 1900 baht, done at Chaengwattana, counter J.