How long in advance can an Imm-O visa be extended, especially during COVID-19?

Seeing as your profile says you're in Bangkok we will go with that premise

In Bangkok you USED to be able to apply for a new extension on a Non-Immigrant Type visa when you had 45 days or less left on your stamp

HOWEVER due to covid concerns out at Chaengwattana Immigration office you can now only apply for an extension when you have THREE days or less left on your current stamp :O

So no you can't extend earlier, in fact the window of opportunity for when you can apply got considerably smaller :(