You can adjust your investment elections as often as you would like. There are no minimum cash requirements or time limits.
If you are investing with TD Ameritrade, please note that investment fees may apply.
If you are investing with HSA Guided Portfolio, there are no additional investment fees outside the annual 0.50% annual management fee. If you currently have pending transactions, you will need to wait for them to be completed or cancel them before making a change.
Be sure to consult with a financial planning and/or tax professional as needed to understand your options.