I have not been able to find Davis Documentation telling how often these uploads are done.
I'm not aware of any Davis documentation that discloses this publicly. But in practical terms, the user can obviously only control those parameters that can be accessed via the wl.com web interface pages and there are no settings to control forwarding frequency to third-party website targets. So the only available upload times are the default ones.
It's difficult to see how WLIP, for example, is ever likely to be able to support WURF, at least not without a major firmware upgrade. WLIP uploads its current conditions data to wl.com every minute (as documented by Davis) which wouldn't chime with my definition of RF, though I guess some users might still prefer every minute or every 5 minutes to the 15-minute interval.
It's worth remembering that wl.com caters for uploads from all of WLIP, WL subscriptions and Connect sources and so eg a Connect even on a 5-min plan is never going to deliver RF data. But the point is that different data sources can have different upload characteristics.
Finally, wl.com has had only incremental enhancements over the past 2-3 years - the focus seems to have been on backstage improvements to make the service as robust as possible while scaling up to a load of 20,000 users and more. As far as I can judge, this seems to have been done pretty successfully. But I'm sure there will come a time, hopefully in the relatively near future, when the development attention will switch to the service itself and the range of features on offer will be extended.