For me, anything non-client related is non-billable. Like Lee, my non billable time would be what I spend marketing, networking and working on my business itself. (For example, I am posting here - this is non-billable

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Also, there may be negotiated circumstances in a contract with a client where something is non-billable. For example, someone may not charge for each and every phone call to/from the client or something like that.