While it does depend on the environment (especially heat) and hours in use, we are talking about vintage Macs here. Simply put, expect the fluid filled electrolytics to last 15 years before they start to noticeably degrade in your vintage computers. Solid tantalum caps or the ceramic chip capacitors used all over the logic board will last as long as the logic board (60 years?). And while it is true that the tiny SMD type electrolytics tend to leak more visibly, the fact remains that non-SMD electrolytics also leak or otherwise experience diminished capacity after more than a decade of service. That is why when I recap my SE/30 logic boards, I take the time to also replace the two large axial caps as well. Anything less than a full cap replacement is not a complete job. And an incomplete job may result in unexpected operation from time to time.