Considering daily usage of the Surface Pro 2 (not gaming) but browsing, painting, drawing apps etc. for average of 4 hours and charging it at 10% each time per cycle. how long do you think the SP2's battery will last until the unit becomes near unusable? Again, I'm NOT knocking on the fact that the SP2's battery is non-replaceable but just want to know how many years I can expect to keep this device going. Also, how many years do you think by YOUR usage pattern will the SP2 will last?