"Peak plasma concentrations of prednisolone are obtained one or two hours after administration by mouth, and it has a usual plasma half-life of two to four hours. Its initial absorption, but not its overall bioavailability, is affected by food."
The half life refers to the amount of time taken for the levels of the initial dose to decrease in the body by half Ie 40mg in reach peak concentration of ~ 40mg in an hour. By 4 hours in the body there will be about 20mg still active, by 8 hours ~ 10mg, 16 ~5mg
Prednisolone is extensively bound to plasma proteins, although less so than hydrocortisone (cortisol).
"Prednisolone has a biological half-life lasting several hours, intermediate between those of hydrocortisone (cortisol) and the longer acting glucocorticoids, such as dexamethasone. "
It is this intermediate duration of action that makes it suitable for the alternate day administration regimens that have been found to reduce the risk of adrenocortical insufficiency, yet provide adequate corticosteroid coverage in some disorders.