It would be interesting to know, when a conversion occurs, how many days ago the visitor landed on the website for the first time.
This is not trivial as "Days since last visit", because we don't know when a conversion last occured on the client side, and this would require a new lookup on the server side, to select the time of last conversion for this visitor.
This would maybe require to implement the 'persistent visitor table' discussed in #1434
Proposed day breakdown similar to GA (which I think is relevant here):
Number of days until conversion 0 1 2 3 4 5 6 7 8-14 15-30 31-60 60-120 121-365 365+
Milestone 1.8.x Piwik 1.8.x deleted