This is exactly why he makes a habit of not thinking about this stuff, until he's forced to by troublesome AIs who forecast him a future that's far, far too possible.
He gives Spike a curious glance because that's definitely a new insight into how Spike feels about him, but mostly he's just clutching at the actual advice in there. The 'enjoy whatever you've got while it lasts' advice is how he tries to live anyway, but how to end it... "Part on good terms," he says slowly, letting this sink in and then nodding, looking up at Spike again. "Yeah. Control the parts we can control. Recognise when it's going sour and make the decision to part then - and then maybe he would take me back later after all?"
He's still trying to find a way to make that part work. The part where he would want an open relationship and Ianto would want him to leave before sleeping with anyone else.