In one of my projects which last 60 weeks (similar to yours) my team has worked in 4-week sprint and it has been going well.
However, to answer your question: There is NO rule of thumb on reasonable number of sprints. I have been managing many projects, currently 5 projects at the same time. Some of them last for less than 10 sprints, some last 10-30 sprints, and some particularly long projects take > 50 sprints. And each project can have different sprint length (1, 2 and 3 week).
In Agile group where I participate, many teams choose 30-day period, while many teams choose 2 weeks and 3 week.
So how do I determine the sprint length? The decision is made after considering: Estimated length of project, nature of product, nature of your team.
1) Estimated length of project.
The estimation needs not to be precise 100% to give you a sense of how long the project will last. If it last more than 6 months, it makes sense to consider 3-week length or 30-day length.
2) Nature of product.
You need to ask yourself:
- Does this product need to adapt frequently to market/real users’ need to make value for stakeholders? If it does, you should shorten
the sprint length and give more frequent workable version to get
- Technically, is it possible to deliver product in short iteration? As you know, short iteration is encouraged, but you cannot do it 100%
of the time due to technical difficulties. In some project we need to
implement complex algorithm for example, before showing it to
3) Nature of your team
Does your team work most efficiently in 1-week sprints, or in 30-day
sprints? If you have worked with the team on several 2-week sprint and
you feel that they work well, why not consider it?
You may have the temptation to try 1-week sprint, but keep in mind that the actual work of coding is reduced, due to the (almost) same amount of effort spent on Agile (meetings, communication, etc..).
In my current projects, for maintenance projects we often choose 1-week sprint and for others, we are working efficiently in 3-week sprints, and 4-week sprints, for to share personal experience.