It varies considerably depending on the individual and how much training they have put into developing their psychic abilities--and even the particular incarnation.

We're all trained to shield our minds, to avoid listening in on others and blocking others from listening in on us. Telepathy between Gallifreyans is generally easier than telepathy with an alien. I'd say the majority can establish a psychic connection with a fellow Gallifreyan with merely the use of a triggering word. "Contact" is what we're trained with. But creating a link with an alien, or creating a link against resistance, usually requires physical touch. There are some Time Lords, however, who are particularly adept and can hypnotize from across a room if needs be.