Here are the average lengths of pregnancy for some mammals, in alphabethical order:
Ok I realize that an interval isn't an average value, but I didn't come up these numbers myself; someone else did.

Mammal                        No. of days
------------------------------------------
Camels                        406 
Cats                          62 
Cows                          280 
Chimpanzees                   237 
Dogs                          62 
Dolphins                      276 
Elephants (African)           640 
Ferrets                       42 
Foxes                         52 
Giraffes                      395-425 
Goats                         151 
Guinea pigs                   68 
Hamsters                      16 
Hedgehogs                     35-40 
Horses                        337 
Humans                        266 
Hyenas                        110 
Kangaroos                     40 
Lions                         108 
Mice                          21 
Minks                         50 
Monkeys (rhesus)              164 
Moose                         235
Opossums                      13 
Orangutans                    245-275   
Pigs                          113 
Rabbits                       32 
Rats                          21   
Reindeers                     215-245 
Seals (northern fur)          350 
Sheep                         148 
Skunks                        62 
Squirrels (gray)              44 
Tigers                        105-109 
Whales (blue)                 315 

reference: ne.se, supermemo