You should probably shoot for at least a few minutes of practice every day. The best time of day to practice depends upon you and your natural makeup. The general rule of thumb is to practice during that part of the day when you are at your sharpest. IQ is not important so long as it is somewhere around normal. If you are understanding at least most of what you are reading here, you are smart enough to do well with self-hypnosis.
Concentration does have a bearing on the various levels of hypnosis.
Better concentration means better and quicker hypnotic states. But if your concentration is poor, don't let that stop you. Self-hypnosis practice improves concentration. Age also has a bearing on hypnotizability in some very general ways. The very young and the very old do not respond well, sometimes not at all.