Generally, you can keep it in secondary as long as you'd like - as death said above. A barleywine for example might benefit very much from bulk aging 6-12 months, who knows...
However, be careful if you try to go like 6 month+ in a plastic bucket (i see thats what you are using). Although they shouldn't be too O2 permeable, they are probably more O2 permeable than an airlocked glass secondary, a pressurized/sealed keg, etc.
Roaring Bull Brewing Co.
Currently Consuming (HB): Apfelwein on Tap Troegs Hopback on Tap; Craft Bottles
Fermenting/Conditioning: Up Next: Hop Trio American Wheat, Lake Walk Pale Ale
In Planning Stage: Farmhouse Saison and Something Oaked.