Tipping

For a group tour of most of a day, for a great tour, I usually give the tour guide 10-15 Euros and bus driver 5-10 Euros for two person.
Less for a half day tour.

For a tour of several days, usually give a bit more per day, since the tour guide does more than just the touring stuff.

For public tours, depending on how good it is, 5-10€ is fine. If the tour was not professional or the guide was boring, you do not need to tip anything. Tips are always of your own free will and never a must.
For private tours, it depends on how many hours, have they met you at your hotel and brought you back, did they do extra research on their own time for your tour to tailor it to your wishes or special interests, or maybe not any of the above. Again, tipping is only if you thought it was a great tour, and were more than satisfied. If you really enjoyed it and got a lot out of it, tip what ever you like - 10, 20, even 50€ or 100€ if it was an all day tour and it was exceptional.