"Reasonable" is whatever it is for >you< and what you wish to accomplish whether it's your 1st or your 20th MC. The notion of this being your first MC plays no role whatsoever in what you decide. Too often people use their first time as an excuse to stop. Ignore that line of thought and if your are up to it and circunstances allow by all means press on.
On my 1st I thought that I'd do 2 weeks and I was feeling so good the second week that I continued to 40 days. Health and cleansing are generally why we all go into the MC but I can tell you the mental aspects of that first 40 dayer was by far the coolest thing I have ever done. It planted the seed for a life changing shift of consciousness.
Looking at it from a pragmatic view the reality is that everything in our system is chemically based including every thought we have. When you fast for that long you are effectively changing your body chemistry and literally you are not who you were when you started. It affords you a look at life from a totally different perspective.