Major donor prospect conversion rates

Hi there

Relatively new in post, I am currently writing a major donor strategy. My organisation has received ad hoc donations from high value givers in the past and is well networked, but has never had a focussed programme. While wishing to demonstrate the potential for significant growth in this area, I also wish to set realistic expectations when presenting my strategy and income projections.

Is anyone willing to share their own experience of conversion rates for warm and cold prospects and also a sense of the timeline for conversion for cold prospects? My understanding is that it's usually in the region of 18 months, but I would appreciate hearing from anyone who has achieved success on different timelines.

Thanks

Sally