Any minor increase to your activity level is a benefit to you but honestly, it depends on where you're at in your fitness level and what your goals are right now.
If your wanting more endurance, higher intensity, or are training for a marathon than the longer you can go the better. Now, if you're just looking to boost weight loss and burn some calories- its not how long you can go for at one time but how much you did overall, time and intensity.
The current recommendation according to the American Heart Assoc. is that you get at least 30 minutes of excerise at a moderate level 5 times a week or 20 minutes intense 3 days a week. (The links take you to the AHA, you want to be really, really in the know, read the link on the AHA page, it takes you to a journal siting the updates put through regarding the change they've made to the recommendation)
And they also state that is more than okay to split the times up (though if I remember right they recommend it in 10min increments at least). Just make sure your working out at a moderate level if your just starting out. My health coach taught me this technique: If you are working out so hard you can't talk, that's intense, if your working out to where you can talk, but can't sing...that's moderate. She also informed me that is considered fine to divide up your times and some nutritonists/diet experts are starting to recommend shorter but more frequent bursts (that way you don't work out for an hour than sit on your butt all day I guess, I call it being sadistic, LOL).
You can always use an online exercise calorie counter to figure out how long and at what intensity you need to maxmize your effort.
Hope this helps