If we assume a first order rate law, then we can use the integrated rate law equation

ln ([A]t/[A]o) = - kt

where [A] are the concentrations at time t and time zero, k is the rate constant, and t is the time. While we show A as the concentrations, we can use the amounts of A in any units as long as the units are the same. In this case, we can use the dosages (mg/kg) as the concentrations.

ln (25/5) = - (0.00426 min^-1)t

t = 378 minutes

The units need to be in minutes since the value of the rate constant was in inverse minutes. If we need it in a different unit of time, we can then do conversions to other units.