Regarding the useful life of impressions: Sensaphonics (and almost all our Gold Circle audiologists) uses the pink, and we have a great track record for accurate fit with our clients.
Impressions do exhibit slight shrinkage over time. Generally, we will not re-make products with impressions more than 18 months old. However, your ear canals can also change. Physical maturity can be a factor. Significant weight change (up or down) can cause your canals to change size/shape. In short, there are only guidelines and YMMV.