How Much Magnesium Citrate Should I Take For The First Time For Chronic Constipation?

I am a female, 59 years of age and I have chronic constapation, I take medication that cause a lot of dehydration or certainly severe dry mouth. Like Allegra 24 HR D, Topamax (which I was told I would need a stool softener).. I Sometimes take Allegra 12 HR D when I can get by with that. I think only taking the stool softener every 7 days is the right thing to do so that it would be effective. So my question is how much Magnesium Citrate could I take the firist time to see if it is helpful. By the way I don t have any tummy pains, and basicly get over any bad feelings fairly well.