you can certainly reuse it but logic dictates that everything in there grows at different rates so what you wind up with isn't going to be what you started with (it could be better, but i don't know, i've never used roselare).
as for how long to leave it on, historically, flanders reds are not left on the cake like lambics... that being said, my flanders has been on the yeast cake for a while now (3 months?) - i should probably rack it to the barrel soon...
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