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Is it smarter to buy a small amount of an ETF every 2 or 3 months, instead of monthly?
By not timing the market and being a passive investor, the best time to invest is the moment you have extra money (usually when wages are received). The market trends up.  $10 fee on $2000 represents 0.5% transaction cost, which is borderline prohibitive.  I would suggest running simulations, but I suspect that 1 month is the best because average historical monthly total return is more than 0.5%.