Surely it depends upon how good you are. If you haven't done the groundschool you are wasting your time starting to do the flying. It is probably better to concentrate some of the flying, an IMC course over a long period of time is not a particularyly good idea. Try and go for four or five lessons without too large a break. Another factor is what aids are available at the airfield you train from, if you have an ILS and an NDB it can be quicker than having to transit miles. Many people do just 15 hours including all those from the USA! Some do more!