Calculating Hohmann Transfers
Kepler's third law, T ? a3/2, can be used to calculate the time required to traverse a Hohmann trajectory by raising to the 3/2 power the ratio of the semimajor axis of the elliptical Hohmann orbit to the circular radius of the Earth's orbit and dividing by two (for one-way travel).
For example, call the travel time for an Earth-Mars trip T and the semimajor axis of the Hohmann ellipse a.
a = (1 AU + 1.5 AU)/2 = 1.25 AU
T = 0.5 (a / 1 AU)3/2 years
= ~0.7 years = ~8.4 months