Once a TARDIS has dematerialized
and the Epsilon Coordinates are input the ETA will be displayed on the console.
Since the Artron Mainframe usually calculates the Epsilon Coordinates,
the Estimated Time of Arrival is based (in theory) on the space-time distance covered, and the efficiency of the Epsilon calculations. The TARDIS also uses data from the
Mean Free Path Tracker to plot the ETA.
A TARDIS travels forwards in time faster than backwards because it isn't fighting the flow of time and, in general, the larger the temporal jump the longer the flight.

However, because of difficulties in creating a safe path through the vortex,
physical distance has little affect on travel time.
A trip of 1 mile often takes longer then crossing half the universe.
While a TARDIS journey can take 9 weeks, in practice, it is rare for a trip to take longer then two days.
ETA also varies on the current state of agitation of the Vortex.
Some operators claim the "mood" of the ship affects the ETA.
This claim could have some basis in fact.
Being four-dimensional beings TARDISes actually prefer travelling in the Vortex to inhabiting the Multiverse. However TARDISes have Curiosity Circuits.
Without these circuits a TARDIS would never leave the Vortex.
Despite all these variables, the displayed ETA will be accurate to within 5%.
It should be noted that the ETA display will not be updated if the TARDIS's course (and arrival time) is altered by some outside force.

Regardless of a TARDIS' current position in the Vortex a flight to Gallifrey never takes more then 10-30 minutes. For example, it takes 10 minutes to get to Gallifrey from 21st century Earth.

It is possible for an Operator to choose a "long" route (and thus extend perceived travel time) through the Vortex. When the total power is reduced a TARDIS's trips through the Vortex take longer.