There should be a guarantee to sit in the chair for 18 months

Leadership positions within a church transition.

Senior ministers.
Assistant minister.
Youth ministers.
Children's ministry workers.
Family ministers.
Pastoral care workers.
Secretaries.
Head elder/Chair of Church Council.
Treasurer.
Cleaner/Propert warden.

For any number of reasons there's a turnover of people. It's unavoidable.

But the timing isn't always unavoidable.

Ideally, for two vital lay-positions, their occupancy should go unchanged 18 months either side of an important change...

The chairperson of the leadership team/church council/elders and the treasurer should be established and committed while a church gets a new senior minister.

Why?

Because the new minister will need these two for a variety of information about the (true) state of the church and the culture of the congregation, putting aside any pastoral support these people can provide.

With these two offices assured, the new minister is spared the stress of advertising for a role, and potentially fielding questions, about a role they're somewhat unfamiliar with in that specific context. 

Furthermore, this reduces the workload and stress from a new minister if these offices are unoccupied for a period of time.

If nothing else, this assures any potential minister that a prospective church is adequately prepared for an incoming minister and that the key people in place between ministers will stick around in the immediate future.