The only study done on Epsom salt baths (cited by Paul above and in my question on Epsom salts that you answered) had the subjects bathe for 12 minutes, and all of them raised their Mg and SO4 blood levels significantly after only a week of bathing, so you should be able to absorb enough Mg in that amount of time, but I think bathing longer will allow you to absorb more, although I have no idea how much more. The question I have that the study doesn't answer is how long would it take for one to increase one's Mg levels if one only bathes in Epsom salts once or twice per week? The subjects in the study bathed in it every day, but that isn't practical for me and many other people due to financial and time limitations.
Btw, if you still are looking for a cheap source of Epsom salts, you have to buy them in bulk, and I bought them here for a ridiculously cheap price (25 lbs. for $23.75, which comes to $0.95/lb. plus free shipping!): http://www.saltworks.us/ultra-epsom-salt-wholesale.html
I am not going to even think about comparing that cost/dose ratio to even the cheapest oral Mg product, because it would make me aware of just how much money I could have saved using Epsom salts instead of oral Mg supplements for many months.