I have all the Manhattan GMAT materials including the online sessions available to me, but wasn't sure if that was the best route to take. I admit I did not study prudently hence the low score so I don't blame the study materials at all. I likely only did 5-10% of the OG problems. They offer Kaplan courses live in my area, but wasn't sure if others have had a positive experience. I read the reviews, but just posing my specific questions to see if folks have thoughts.

If you have the M - GMAT materials on hand and access to the sessions you should start there! Online sessions can be convenient and work well for many people. I am not sure if you have live sessions available or pre-recorded. I love teaching the live sessions online and pre-recorded sessions like the Veritas On-Demand sessions are very popular. So you should try the material that you have. You can always spend more money later if you need to.

Is 5 months perhaps too long to study?

Not necessarily too long, however I would advise you to set a goal of taking the GMAT again in 2 to 3 months. That will give you the option of postponing if you have to. But if you wait too long to re-take pressure can build as you start to think that this is your "only shot." Also you can get burned out.

So my advice is to put together a study plan for yourself that is between 2 to 3 months and use the materials that you have on hand! After maybe 6 weeks you can take another GMATPrep practice test to see where you have improved.

Oh and to answer the question in your subject line: Yes! You can go from 500 to 600 + in 5 months (or less).