I have my HSA with HSA Bank and I use the TD Ameritrade option to invest as well. I keep about $3.5k in the bank account because I do use the debit card for prescriptions, co-pays, etc. I keep the balance over $3k so that there is no monthly charge, although I think it only comes out to $24/year I believe if the cash balance drops below $3k. I should maybe keep only $1k or so in that and invest the rest. Anyway, I have all my investment account in VFIAX (S&P 500 Index). I'm on a little shorter timeline, maybe 10-15 years when I expect to need it, but VFIAX works for me. As far as when to buy, what I do is usually purchase every 3-4 months which, in my case, is when I accumulate approx. $1,000. I really like the Ameritrade option to inve...