would be nice to get a calculated metric with time operations. i think this would solve the problem.
right now, i can't see any simple solution to get your answer. instead, you could create a bunch of segments and create a complex calc. metric to get your answer.
idea (for someone with enough time): you create a segment with different behaviours, eg:
1) visitors with "lead submission" in "first visit" => 0 days
2) visitors with "first visit" then within 1 day a "lead submission" => 1 day
3) visitors with "first visit" then within 1 day "no lead submission" then within 1 day "lead submission" => 2 days
4...n) just the same as 3) increasing the timespan of the "no lead submission" (make this for example until 30 days)
y) same as last segment above but the last condition of the "lead submission" is without timeframe. this will fetch all the rest of the successful subscribers. give them a high value for success days (eg. 60)
z) all visitors with "lead submission"
after having the segments you just need a calculated metric which counts the number of visitors for each segment (2-y) multiplied by according days and divide everything through visitors of segment z) (=all successful visitors).
this should give you sort of indicater for the average time ... if someone tries this, i would be happy to hear any results and if it works as described 
regards
Urs