CBT therapy: once per week.
DBT therapy: once per week plus group therapy once per week.
supportive psychodynamic: once every fortnight (mostly because of time constraints, i think)

now i'm seeing my t every friday and every second tuesday. he said twice a week would be optimal but i said something about how it might be better or it might be worse and so he offered me this comprimise and we can see how it goes.

i think that twice a week is optimal for personality disorder type stuff. it can take a while to build up trust and while you can't circumvent that by seeing each other every day or whatever you can certainly help it along by seeing each other twice a week rather than less often. it also helps the day to day stuff not take over the sessions.