It may be possible, even if you are new to making armor / using the materials, but you definitely want to start as soon as possible and give yourself a lot of time to work on it each day.
One way to determine how long it will take is to create a work schedule, giving yourself at least a week of breathing room before the convention. Lets say you can work 5 hours a day on the costume, set a specific job for that day that will take about 3-4 hours and see if you can fit the entire process in that time frame. Remember it's not just making the base of the armor, but painting (and allowing the paint to dry) and working on how each piece will be attached (strapping / velcro / etc)
Why the shorter time amounts? That last week (or more) will allow you to test the costume and fix any minor issues, and add any additional details - plus it's never fun to stress about a costume right before the con! The reason I say 3-4 hours out of a 5 hour work period is because you may need to go to the store to get more supplies, restart what you were working on (in case of mistake), or have it interrupted (friends, family, the need to eat etc). If you do finish in the 3-4 hours it gives you an extra hour to catch up / get a head start in other areas!
Hope this helps you out!