It is critical that elected politicians have a direct, immutable dependency on the success of the private sector because they do control the rules. The same is true of career bureaucrats but they, at worst, interpret (IRS) and / or apply the rules (all others for the most part).
The quantitative limit on terms of service is open for discussion. My inclination is 12 years in any one section (House or Senate) and 18 years in total.
The guiding rule should be that their pensions be available on the same age basis as Social Security, and never exceed 50% of their highest pay grade adjusted for inflation. Inflation adjusted earnings should be on the same basis as the SSA (Social Security Administration) does in calculating AIME (Average Indexed Monthly Earnings), the base for calculating OASDI benefits.
Similarly, health benefits provided to them should be Medicare only in retirement. While active medical and dental plans should be subject to the same rules that apply to private sector plans.