Its all in the mind – and differenet peoples pain thresholds vary. Mine seems to be 5 hours. Am in the process of having a full sleve right arm – planning it for some years now. Had 3 x 4/5 hour sittings (2 to go!), all fully enjoyable. No discomfort apart from inner arms. Its a throughly enjoyable experience if you have the right tattooist – it funny how chatting about life whilst he/she is doing it removes you from the supposed 'pain'. Very addictive experience.
There is anesthetic creams available (google it, Boots do a specific one but it gets abused by Junkies so its hard to buy now) – they can be rubbed into the area to be tattoo'd prior to the visit. I've never tried them as was warned that they can affect how the skin accepts the ink.
You only live once! – and its a fantastic experience.
Think very wisely about design – a good idea is to have you're design on the wall for 6 months or so. If you still like looking at it after a few months, You will then soon know if you love it enough to have it on you're body!
Enjoy the ride…