It all depends how long you fly on each rating. If you have flown many hours in a 3-axis microlight, upgrading to SSEA will most likely require only the minimum 3 hours dual, a cross country flight, the GST and the only JAR-FCL PPL exam not done on the microlight course, Aircraft General and Principles of Flight.
If you then fly your homebuilt SSEA for at least 35 hours, to upgrade to a JAR-PPL you need 20 hours dual with a JAR qualified instructor, of which you have already done at least 3 on the SSEA course. You already have the exams under your belt, so if you are good, as little as 17 hours dual may be all that is necessary before the GST.