I wouldn't wait until you cramp to take it ... BUT when you start to get some symptoms of cramping, then take it. ( i would presume you get symptoms well ahead of an actual cramp ... in the rare case that I cramp I know its coming 30-60min ahead of time) At that point it is not too late. If you feel inclined you can just keep taking salt at the present level but it is not necessary.
Also, you should try to find the actual source of your cramping. Cramping is not coming from a sodium deficiency. More often than not it has to do with muscular and neuro-muscular pattern use / over-use / or under trained. Probably good to take a step back and look at
- training
- effort levels in training and racing
- bike / cleat position / technique
- intake levels of calcium & magnesium
For example ... I had an athlete who was always cramping late in bike rides and kept taking more and more electrolyte capsules. I decide to go for a ride with him, rode behind for while and just observed. I had him change is pedaling / shifting technique (essentially shift early before hills & rollers and ride those rollers a little lighter than he had been and use less force) ... 9 days later in his IM race in high heat and humidity he never cramped once, and he took only 25% of the electrolytes he would have normally supplemented through capsules.
Think big picture ...
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Dave Latourette
http://www.TTENation.com