how much would a timing chain strech over 250000 km?

How longs a piece of string?

Are you talking 6 or 8?

The easiest way to check is to lift the chain up off the top of the timing gear on the camshaft. On a new chain it will hardly lift up. On a worn chain it may lift up 3mm or so.

Where is the timing mark on the camshaft sprocket in relation to the head when the engine is at t/d/c on no.1? It can be up to 3mm above the head surface. That will give you a guide to how far it has stretched as well.