What joint space are you caulking? Between what part of the window and whatever?
What have you been using?
Most of the "lifetime" acrylic latex and/or silicone-based (paintable type only for me) caulks last a fairly long time (>5 years) if they are applied correctly (clean, dry surfaces).
Several reasons caulk can fail. Wrong product for the material being caulked. Joint surfaces not properly prepared. Joint not properly designed.
Here's a link on joint design. Silicones work well on smooth surfaces. Urethanes work on masonry and wood. Acrylics work on wood if surfaces are cleaned and prepped properly. You can't apply caulk on top of old, failed caulk. Read the label for instructions on joint prep.
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