The same time frames that existed when humans manually traded.
Thinks of it like this. When I put in a trade at human speeds based on business fundamentals, I'm not looking that the millisecond by millisecond prices, I just put in a price I am willing to accept and if the market reaches that price I get execution. HFT makes that easier and more efficient across markets by ensuring prices are converged rapidly.
How do you think it makes it harder or worse? If I put in an order to buy at $x on a particular market because I think the stock is worth more than that for business reasons, what is it about the existence of HFT that is a problem for me?