yes very true Kathy i had mines with a heater and a powerhead for about a month and a half testing for Ph till it dropped to the same ph level that it was before adding the rock to the bin ...It also depends how much rock you are talking about the more rock the more time...But if you have a big system it wouldn't effect if you was placing 1 or 2 DIY Rocks after a week of curing..The biggest issue I've heard (from a friend who made most of his own LR) is it needs to be cured for a long, long time (months!)- outside of a tank and with constant water changing - to get the pH to drop. Part of one of the ingredients is lime which has an extremely high pH. It also tends to crumble in time if you do not use the correct mix of ingredients.
Otherwise - it's a great way to make the shapes you desire