I'd go ahead and start now if you have the time. As to your question about retention, that can vary from pilot to pilot and has a lot to do with how much time you get now, and how well you take to the instruction. Most likely, if you reach the point of solo, you will be able to retain most of what you learned through a few months' hiatus, but it would still be a good idea to have a more experienced pilot on hand at the beginning of next season just in case you do need a bail-out. Many modlers take several months off from flying every year during the winter months to build, and may not touch the sticks for 4 months of the year or more. Most come back rusty, but after a few flights are back to where they left off.