On-demand consulting is available rounded up to the nearest quarter hour.
Hourly billing is rare. Rates are discounted for reserved instances and projects (see below).
Most engagements, regardless of scope, end up being structured as reserved time equivalent to one or two full days per week.
This is for when you have a clear project and/or consistently need work done. You'll get a discounted rate and guaranteed availability. This generally involves digging in to your objectives and time frame, then working backward to estimate the level of recurring effort required.
Reserved time is billed monthly up front. Once engaged, work tends to be organized into weekly objectives.
Groups (e.g., VC portfolios, incubators) can also reserve time with a master agreement and delegate it to companies based on need.
If you have a specific project in mind, or want to create one together to meet a stretch goal, let's talk about how I can help.