The phenomenon of 'tolerance' usually applies to medications/substances that act on reward centres in the brain and have addiction potential. Escitalopram is not one of those medications. If it seems like the effect of escitalopram is decreasing, it may be the scenario that a higher dose is required or that the symptoms of the original condition it was prescribed have returned due to any reason (return of symptoms may be due to a social stressor unrelated to medication also)
Next Steps
If it feels like effect of escitalopram is decreasing - consult treating psychiatrist for reassessment