What is your estimation of the lifespan of this republic?
This form of republic is very new. While the Romans had a form of republic it certainly did not take this shape.
If you apply the strictest of standards and say it is only lives as long as it does not change then the first US republic died in 1828 for sure and it can be argued that it died even earlier.
Theoretically as long as it remains flexible the lifespan could stretch to infinity.