Borax causes men to produce more testosterone and women more estriol. For those of us older than 40 or 50, that would seem a very good thing.
Taking 2 or 3 days a week off the borax is to (supposedly) allow your body to adjust and your hormones to balance.
Several people take borax continuously and don't seem to suffer any ill effects. As usual, do your homework and make responsible choices. However, if taking borax 4 or 5 days a week gives you the results you want, why take it for 7?
Many health professionals advise taking even your regular supplements only 5 days a week (or 3 weeks on and 1 week off). The reason for this is because anything we take continuously may attenuate -- become less effective or even ineffective. This happens all the time with prescription drugs. People who were taking MSM every single day and getting great results suddenly were not: The MSM just stopped working for them after 6 months, a year, or in some cases many years.