When does a prospctive client become an assumed client?

[Maria Teo]

Question for Real Estate Agents:  At what point/after how much interaction with a prospective client, do you assume you have an unspoken agreement to work together? 

I'm talking about the open-ended interactions, not situations where you've specifically spent your time for an individual. 

For example:

a) If a prospective client encourages you to send them your listings?

b) If a client shows up to your open house after seeing one of your listings?

Yes, technically you have to have the contract, but really when do you think a commitment can be assumed?