The bristles of a normal brush or the normal toothpaste are not abrasive enough. However, if you soaked your teeth in something acidic, waiting 20 minutes may be a good idea, but even then we are talking "maybe" a couple microns. What is commonly called toothbrush abrasion is incorrect, it is usually a bite issue. Even trained dentists get this wrong. If you are waiting an hour, that is still better than most. You don't need to wait, but it doesn't hurt.What you MAY be referring to is the situation after an acidic event. Like when sick, if one vomits, the stomach acid softens the enamel. The taste is terrible and one is inclined to brush in order to remove the taste. However, this CAN remove some microns of enamel. Once or twice is not a big deal, but a bulimic, a person that purges several times per day, will have significant damage to their teeth, both in the erosion from the stomach acid and the brushing away.