I charge most of my income as a monthly subscription. We switched years ago and couldn't be happier with the arrangement. The clients like it too.

What we found was that clients wanted stability, and they don't really comprehend the hourly arrangements anyway, no matter how we explain it. They just want to be quoted a price they deem fair, and regular enough to budget.

We still do it based on hours. I'll say I've calculated their needs based on our ample experience scoping projects, and I believe they will need X hours per month from me on avg. So I recommend a monthly budget of Y.

We still track hours like it's an hourly project, and report on billing usage over the periods. But the client thinks of it as a monthly subscription to get the services they want.

I will seek to adjust the billing periodically, about once per year for stable relationships, or more often if it's a new relationship with changing needs. I'll say we've reviewed the hours and can now correct the monthly billing to match the updated needs. This is also the time I'll alert them of rate changes for the following period.