How long does a syrup pod last?

It depends – the stronger you set the flavour, the quicker the pod will run out.
Something like this:
Level 1 → approx. 5 L
Level 2 → approx. 3 L
Level 3 → approx. 2.5 L
Level 4 → approx. 2 L
Level 5 → approx. 1.5 L