Reaching the limit of filtration often is accompanied by a change in taste, which is most commonly characterized by the re-emergence of the taste of chlorine. One can obtain pseudo-accuracy by noting the number of refills usually consumed over a short period of time and doing the math to estimate the number of months expected for the gallon rating of the filter. For a 30oz. canteen at a rate of 4 refills per week, this would translate into approximately 1 gallon per week. For a 100-gallon filter, this equates to about 2 years. Obviously, this is probably a very conservative estimate.
The new filter does not fit the old canteen.
The old filter style is no longer being manufactured. If it is available, it will appear with the "Click to Buy" button above.
If it is no longer available, the only other option would be to consider the 38oz. Canteen at http://www.naturesalternatives.com/pres2pure-canteen-advanced-filter-water-bottle-38oz.html