I will throw in my 2 cents. What others have said is very useful information. I would recommend if others are involved in the specification process (as opposed to you spec and building report yourself) that you set their expectations to the limits of Crystal in the development of a report. To do this, I recommend that as part of the report specification (for complex reports) you have a prototype phase. With all this said, I don't believe it answers your initial question... Here goes my response.
A crystal report can have varying degrees of complexity. I recommend if you have several reports to spec out that you classify each in a high level category of complexity (low, med, high). Low complexity reports may be pretty easy to estimate durations for development. The medium and high complexity may require that you have a "formal" specification process. This will of course add a good degree of uncertainty to the estimation. Therefore, I recommend that you estimate the duration of the development in a 3 point estimation spread (best case, what you expect, worst case). Then get a distribution estimate on this based on your degree of confidence (eg 50% or 85%). Then you will have some better estimation.
Other things to keepo in mind. The testing (if done by you, or others) must be taken into consideration. This may be more than the development time.
In general, tyhere is no one formula foir estimating time (that would be a serious error). In stread, look as hard as you can at each, categorize, and appy appropriate project management skills.