Shop says new piston rings required. Am I being conned?

[unfazed]

Difficult to say really as it depends on whether they lap the valves or not. 5 to 8 hours would not be unreasonable.
Much depends on the experience of the mechanic with 16valve heads as an inexperienced mechanic  could spend 2 to 3 hours doing the valves alone.

Head can be removed without removing the engine.