We don't work on the old "spend it or lose it" basis and it's not unusual for even relatively small parish councils to have six-figure reserves. That's good practice if the money is required for a reason, but I've seen several where the funds are allocated to cover an insured risk, so are unlikely ever to be required and others who just attach vague headings such as "highways" or "open spaces" without having any plans to justify this.
I find it helpful to look at this from a pounds per resident point of view and consider the impact of that on those at greatest need in the community. When I took on one of my councils it held unallocated reserves of over £125 per resident and had raised the precept in each of the preceding two years, whilst we had pensioners struggling to heat their homes. That's morally indefensible and I told them so.