it really depends on your individual opiate tolerance and what route yer taking it.. the first time i had morphine IV, it was 5mg, and it knocked me on my ass.. but no tolerance.
Oral morphine is totally useless, you need, even with no tolerance, like 60-90mg to get off on it, and by then itll probly make you more sick than anything.
My gf has a low tolerance, and 15mg snorted will get her nodding, with some nice hallucinations to boot.
personally, i have quite a high tolerance, and lately need about 60mg IV to nod, but thats the reason im taking a little drug vacation.. i feel like shit, and my tolerance is too high.. ive had enough for a while.
morphine is more of a "fucked up" feeling than a euphoric one, but you will feel nice and dreamy if you do enough.. start low and work your way up.