What is the minimum time an S&P 500 index fund should be given before selling?

I don't need funds at this time.

Then there's nothing to do.

If the strategy is "buy and hold" then you keep holding. When you need capital you assess your allocations and potentially sell some of this position. If you think you have allocation issues now, then the decision to reallocate anything would have to include an assessment of all your assets (and age really) not just this position.

Buying the S&P 500 Index is a wise decision. It is a benchmark and you are basically buying the whole market. Not sure what you mean by 'considerable returns', but the average return for 3, 5, 10, 15 years is 9%, 8%, 13% and 7.6% respectively. This data is from Morningstar. To expect much more than about an 8% return over the long haul is probably not realistic. Hope this is helpful.

Brooks