How'd you find that?
The at least just sounds like a minimum to me. Or rather, if he wanted to convey it having a much longer max length, what would he say?
Combined with it being a difficult question makes me think he just doesn't know himself, so gave a non-committal answer. Or he wants to keep it a mystery for fun.
Crazy to think those are probably kids mailing in lots of postcards in the 1980s, excited to interact with a mangaka. Now they're in their 40s-50s, excluding any who passed already.