Ers achau = for ages, am bythefnos = for two weeks. Are ers and am exchangeable or restricted to some expressions or contexts?
'Ers' is better translated as 'since' so the expression 'Ers achau' is literally 'Since ages' which is transposed to 'for ages' in English.
So 'ers pythefnos' would be 'since a fortnight'
While 'am bythefnos' would be 'for (a period of) a fortnight'
But since is used before points of time, and for before periods of time. Is there the same distinction in Welsh?
Yes, so 'Bues i i ffwrdd ers pythefnos' = I was away since a fortnight (ago)
'Bues i i ffwrdd am bythefnos' = I was away for a (period of a) fortnight.