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25 yards, about... Half that 50 yards spool.The thing is I sometimes like to look at the line on the spool. It irks me just seeing the knot there. Tells me I'm running out of line even though I probably still have several yards left. Just me and my comfort zone, but I also used these same reel to fish open water.
http://www.blogsmonroe.com/fishing/2013/01/how-to-spool-an-ice-fishing-reel/I usually can get two or three reels out of 100 yrds/ pony reels of line .I believe I read somewhere that you should use 3x the depth as rule of thumb for line capacity after backing .
Fill your reel to capacity........what happens when you hook a bass,pike,catfish,walleye. Then when you respool you only pull off a 100 feet then put a 100 back on.