How fast does the swimming reflex of an infant disappear and how can I keep it active without going to the pool in the first 6 months of its life?
Since the reflex naturally fades as the baby approaches six months, and it is a reflex, there is nothing one can do to maintain the reflex. It is not a learned behavior or even a consciously controlled behavior. Additionally, the reflex alone does not keep kids safe, it is a reflex only and they can tire easily etc. The dive reflex can keep them from choking if dipped underwater in brief stints, but if done for too long, or too quickly between dips, choking can still occur. You also must be careful about baby not swallowing too much water while in the pool which can put their salt balance out of wack.
In fact, the American Academy of Pediatricians recommends against swimming lessons (for the purpose of learning to swim) until a child is age four, and suggests instead, parent with child water safety programs. In these, a child is taught general pool/water safety, a few techniques to help keep them afloat long enough to get help if needed and just makes having fun with a trusted adult in and around the water so the child is comfortable with the water the goal. (Ages and stages guide to pool play and safety by parent's magazine)
Working with baby so that he or she will graduate from simple and primitive reflex to helping that child get to the edge of a pool, or float well enough to get help if needed, is possible. Unfortunately, outside of having a pool, pond or other large body of water in which to practice (or at least very large bath-tub) availabe in order to work on these skills, I am aware of no way to teach your child swimming without actually exposing the child to opportunities to swim.
Please always keep safety around water and children your highest priority!!
Some resources you may find useful (I did not personally use some of these - we've done lessons at the YMCA):