I believe during the development phase (before you publish the channel) if you submit a feed update it takes about 10- 20 minutes for roku to update everything, sometimes I have encountered it in as little as 2 minutes. Then you go to your roku device, goto system and do a system update check and it updates.. otherwise you have to wait for your device to auto update which could be over night..
I haven't tried to update anything yet once a channel is published, but I assume it has to be "approved" by roku, especially if you have advertising enabled.. The best way I have seen to troubleshoot things is to create a second channel as private without advertising, the updates are alot faster that way, so you can experiement on the private channel once you get what you like just mimic it on your public one.... cuts out the waiting for approval to publishing...
http://www.TVByDemand.com