This is entirely dependent upon your contract. What does your contract state? Does it state hourly billing for hours worked or does it state retainer billing for a monthly block of hours with a "use it or lose it" provision?
The most important thing you can do as an independent/freelance consultant is to make sure your contract spells out in specific details the exact terms of your engagement. This includes things like scheduling, availability, hourly rate, hourly commitment (how many hours per week/month), overages, underages, discounts, after hours work, travel costs, etc., etc. The second most important thing you can do is to purchase professional/business liability insurance.
Below is a boilerplate hourly contract that I use. I start with this and tailor it to the specific client:
By signing this agreement, [CLIENT NAME] (Client) has retained
[SERVICE PROVIDER NAME] (Service Provider) to provide Information
Technology related services for the period [START DATE] to [END DATE],
and agrees to the terms and conditions as set forth in this Agreement.
During this period, Service Provider agrees to provide Information
Technology related services at the rate of $ [HOURLY RATE] per hour on
assignments to be determined by Client. Work will normally be
performed at the offices of Service Provider but may take place at
other locations as required by Client. Work will normally occur
between the hours of 9 AM to 5 PM, Monday through Friday. Unscheduled
or emergency work performed outside of these hours will be billed at
the rate of $ [After Hours Rate] per hour. Scheduled after
hours work will be subject to the standard hourly rate. All scheduled
after hours work must be scheduled a minimum of 7 days in advance.
Payment for these services will be made to Service Provider upon
receipt of Service Provider invoice. A monthly invoice will be
submitted to Client no later than the 5th of the month immediately
following the month in which the work was performed.
All expenses exclusive of normal overhead are not included in this
agreement and will be billed separately. Examples of such expenses
are: delivery services, long distance telephone calls, and all
expenses related to travel such as travel time, mileage, meals, and
accommodations. All travel time to work locations other than Client
office is billable. Mileage of more than 25 miles for travel to work
sites will be submitted as an expense at the current IRS Standard
Mileage Rate. All invoices are payable upon receipt.
All materials furnished by Client will remain the property of Client
and will be returned upon request or upon termination of this agreement.
The results of any and all work performed by Service Provider for
Client will remain the property of the Client. Client may use this
material in any way deemed appropriate.
This agreement shall be subject to voluntary termination, without
cause, by either party. Any rights to compensation or remuneration of
any kind are to terminate at the time this agreement terminates.
Each party agrees to indemnify, defend, and hold harmless the other
party from and against any loss, cost, or damage of any kind
(including reasonable outside attorneys’ fees) to the extent arising
out of its breach of this Agreement, and/or its negligence or willful