It somewhat depends on your usage. We filled our tank last fall and it still reads 100%. Our usage if very low since we only use gas when my wife is cooking and that does not consume much. I normally get it filled when it gets below 1/4 but could probably wait longer than that. If you use gas for a refrigerator, water heater, stove, and furnace you may want to fill it a bit sooner. In a previous unit we could go through a full tank in one weekend when the temperatures got colder.
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