Ok, here's the table I use as a guideline. But really, I use it just to get an idea of what to expect, I usually go by taste in secondary (something you really can't do in primary, btw):
Rule of thumb if small seeds no more than 14 days, everything else but Citrus at about 30 days kind of give up all their flavor, Citrus can go longer.
All of these Fruit additions would take place in Secondary and all are for 5 gallons of mead.
24 30 Days
Blueberries, Cherries (Sweet)
Mild Fruit Character 5 lbs in secondary
Medium Fruit Character 7 lbs in secondary
Strong Fruit Character 11 lbs or more in secondary
Citrus Fruits
Mild Fruit Character 5 lbs in secondary
Medium Fruit Character 7 lbs in secondary
Strong Fruit Character 9 lbs or more in secondary
Pears, Apples
Mild Fruit Character 5 lbs to 7 lbs in secondary
Medium Fruit Character 9 lbs in secondary
Strong Fruit Character 11 lbs or more in secondary
Plums (use skins as well)
Mild Fruit Character 4 lbs to 6 lbs in secondary
Medium Fruit Character 8 lbs in secondary
Strong Fruit Character 10 lbs or more in secondary
8 10 Days
Raspberries - Blackberries
Mild Fruit Character 2 lbs to 4 lbs in secondary
Medium Fruit Character 6 lbs in secondary
Strong Fruit Character 8 lbs or more in secondary
Strawberries
Mild Fruit Character 5 lbs to 7 lbs in secondary
Medium Fruit Character 11 lbs in secondary
Strong Fruit Character 13 lbs or more in secondary
Also, this link is pretty helpful too:
http://www.winning-homebrew.com/melomels.html