I don't grow a ton of stuff in containers but for my vegetables (mainly tomatoes and peppers), I just toss the soil into the raised beds at the end of the season.
We have these big cement planters in the front of the house that I plant annual flowers in and I haven't changed the soil yet, just added a little bit of compost. We bought the house almost two years ago so it's at least three years in those planters.
I also use pro mix but the compressed bales are about $15 here.
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