When to speak less formally?

Say you start talking to someone from here, e-mailing each other and such. Of course you'd speak formally to begin with. But when does it become acceptable to use the less formal language (e.g みる instead of みます)

Fool The World,

That's when the other party starts speaking less formally to you. But normally, people keep using keigo when writing even when you're speaking in casual form when seeing each other in person.

i'm not sure i agree with uco this time

for business acquaintances or people much older than you or that have some kind of junior/senior type of social gap, maybe that is true, but for people the same age i think tameguchi starts pretty quickly.

in my emails i start to use casual language usually after i've known someone a week or so, sometimes faster if the person i'm talking to seems friendly toward me.

but it does depend on age and a number of other factors. i'm in my mid 20's for what it's worth