My cakes always take much longer than the baking time suggested in the recipe. They eventually end up baking fine, but I always have to keep checking them, and also wait much longer. I was wondering why would this be. Could anyone help? Thanks.

Check your oven.  It sounds as though it my not be reaching the temperature you think it is.  Set the temperature and stick a thermometer in it to see if it's reaching the proper temperature.

are you filling them too full? and of course opening the oven door cools the oven so try to keep that to a minimum -- write down how long it takes for a certain batter in that certain pan so you'll know next time --

here's a chart regarding suggested batter amounts and other things:

http://www.wilton.com/cms-wedding-cake-data.html

Thanks for the tip! Let me check that.

That's a very helpful chart. Thanks so much!

and if you use the wet strips -- sometimes called magic strips it can increase the bake considerably depending on how wet you get them

and you're wecome

You don't give us any indication as to just how much time difference there is.  But just the same I bet you oven temp is off.  If it's an electric oven one of the elements is probably not working.   Call for a service check.  Most of the time both elec and gas companies will do that for free.  It's call 'calibrating'.  

I bake at 325F and it always takes a lot longer.   I never go by what instructions may say, I go by the appearance of the cake, and when it looks done, I start doing toothpick tests at 5 minute intervals. 

Usually I have to bake for around an extra half an hour. It's a new oven, so I assumed there isn't a problem with it.

By the way, what kind of thermometers do yo use for oven?

I do the same, but usually its wet from inside so I have to end up keeping it for longer. I avoid opening the oven again too frequently then.

Even new ovens can be off calibration :)  Invest in a good thermometer....do NOT get those cheap store ones w/a pointer.  They are soooooooo inaccurate!  Look for one at a kitchen or appliance repair supply store.  

I agree that your oven temperature is probably off.  I recently moved and had the opposite problem.  My cakes were not baking evenly and the outside would be dry and inside hardly baked.  I bought an oven thermometer and found that my oven is always 15-20 degrees hotter than I set it.  Solved all my problems.  I keep the thermometer in there now.  Also, don't open your oven to much.  Drops 25 degrees or more every time you do.  

I have had this problem with cakes with nuts or fruits (almond cake, fig cake, zucchini cake). The outside would be baked like you said, and inside would sometimes be all wet/partially baked.