I am sure that you can scrape more high for your weed with longer hits, but think about it, is it really a good thing to get in the habit of not only holding your breath, but to combine it with breathing in carbon monoxide (it replaces the O2 you breathe in) and carbon dioxide for a 10-15 smoke session? it just seems like the negatives outweigh the positives in terms of air deprevation, but i guess if you dont really care about health, and just want to get high, go for it.

for me, i hold the hits as long as they are comfortable, which is usually 3-5 seconds max.