Any reason to lager fruit - say raspberries - more than 3 weeks or so on a non-brett beer, e.g., witbier? I’ve never heard a convincing argument why that would make the beer any better…
Using something like fresh cherries, I’ve read that there’s benefit to leaving the beer on the fruit, as the cherry pits over time give the beer a pleasant character independent of the fruit. Personally, I love to use good quality purees (like Oregon) to avoid the racking nightmare as well as after a couple weeks, the puree is fully fermented and infused into the beer.
Raspberries don’t need much time at all, particularly if you freeze them, thaw them and pour them in juice and all or ad Jon says puree them or use a good puree. Bigger fruit like peaches do benefit from longer time.
If you are lagering the fruit on the beer have you already allowed the yeast to ferment the sugars first? Be aware that when adding fruit or purees you are adding additional sugars to the beer which the yeast need to consume before packaging (especially if bottling). I typically will add the puree or fresh/frozen fruit to a secondary carboy and rack the beer on top. i then allow the beer to ferment around room temps (66-68F) on the fruit for about 4 wks prior to cold crashing and packaging.