As much as somebody can do
Definition 1 of 2
Pro Tip 1/3Effort Intensity
Indicates the highest degree of effort or intensity in doing something. 
She ran as fast as she could do to catch the bus.
Pro Tip 2/3Stress Emphasis
Used to stress the effort someone is making, often in challenging circumstances. 
He studied as much as he could do before the exam.
Pro Tip 3/3Limit Expression
Conveys that someone is maxing out their capability or resources. 
I'll save as much money as I can do for the trip.