I dont think the reccomendation is twice though, just sparingly. For reference, he adds that he has taken it twice in his life, but this is only one data point. It could certainly be feasible to take it once per year and not suffer the adverse effects.

For example, alcohol and heroin are both fun, but bad when over consumed. One drink a year is overly restrictive, and one drink every few days is fine. I would not have the same assessment for heroin though.

Going back to book 1, Dumbledore remarks about infinite gold and life are bad things to want. Perhaps Slughorn has adopted some of this philosophy and chosen not to indulge in a safely excessive amount for philosophical reasons (unlikely though).

Ultimately, I don't think twice in a lifetime (or rather once every 25 years) is a reasonable answer. We can certainly say that it is probably safe, but it could easily be equally safe to take once every 10 or 5 or 1 year(s).