Although this is a challenging question, based on hundreds of Kratom users feedback the range below seems to be the most accurate. Kratom effects, when taken in moderate doses will generally last 3 - 4 hours, much like many medications. The effects that Kratom will have on a person involves a lot of factors such as body mass, general tolerance, etc. In any case, one should always use caution and start with a low to moderate dose and work your way up from there.
After interviewing several Kratom users regarding dosage it was found that the starting point was generally 2 -3 grams and then a 0.5 gram increase on the next use (Warning: This increase was not done immediately, but the following day in this interviewees case) and allow yourself 20 - 40 minutes for the Kratom to take effect.
Another rarely talked about factor is the difference between Red, Green and White Veined Kratom. In over half of the users interviewed it was found that many Kratom users either responded better to Kratom users feedback the range below seems to be the most accurate. Red Vein Kratom or the other two, White and Green Vein. It seemed that there are two camps here. The reasons were, according to these users, that the Red Veined Kratom such as Thai and Maeng Da generally offered a higher level of energy and for those that did not respond well to Red Vein Kratom, caused edginess. Others, who preferred the White and Green Vein Kratom generally reported a lower to moderate stimulating effect (Energetic) and higher Euphoric effect.
Another very important factor to mention is that Kratom taken in higher doses will cause a sedating effect. It was reported that, in many cases, the Kratom user had taken a dose which was too high without realizing it and experienced this sedating effect, thus not thinking that the Kratom had worked at all. In fact, the Kratom did work, but unbeknownst to them, they had taken too much and gotten tired instead of experiencing the pleasant stimulation that they'd heard so much about. With that said, if you experience a sedative effect instead of a stimulating (Energetic) effect, you probably took too much and should reduce the amount of Kratom used on your next dose. In cases where too much Kratom was taken and sedative effects were experienced, one must allow 6 - 10 hours before attempting another dose as to allow the Kratom effects to absolve.
If you have taken Kratom and do not feel the effects in an expedient manner, DO NOT take more or double your dosage. Factors such as recent meals can delay the effects, so allow yourself a minimum of 1 hour or longer for the effects to take place. Another factor regarding delayed effects have been reported when ingesting Kratom capsules. Bear in mind that your stomach must also dissolve the gelatin or vegetable cellulose capsules.
If you've taken a low to moderate dose of Kratom and experienced very mild effects or none at all and have waited over 4 hours, you can increase your dose by 0.5 grams. Just because you are over 6 foot tall and 200 Lbs. does not mean that more is better. Upon one male Kratom users experience, he was was 6' 4" in height and over 200 lbs and could not take more than 2 grams of Kratom per dose without feeling sedation. In another interview, a female Kratom users experience, she was 5' 1" in height and 98 lbs and her perfect dose was approximately 4 grams per dose. This information helps to reinforce that moderation with finding your perfect Kratom dose is imperative.
Finally, there have been no reported overdoses from the use of Kratom. If one were to take an exceeding large amount of Kratom, the body would simply expel it (Throw it up). There have been a small amount of reports where people had high amounts of hard narcotics, alcohol, cough medicine and Kratom in their system and had extremely negative effects. For anyone with common sense, including the medical examiner, these effects were not caused by Kratom.
Caution: The Kratom doses above apply to Kratom leaves or powder (not extracts). Peoples sensitivity to Kratom can vary, and Kratom can vary in potency based on the type or where it was sourced from and sometime vary quite a bit, so these dosage estimates should be regarded as approximations only. As recommended above, one should always start with a low dose when experimenting with a new batch of Kratom. It is not recommend to take a strong dose on the first time you are sampling a new batch of Kratom. It is best to take Kratom on an empty stomach when using strong doses (i.e. wait about 3 hours after eating). Those who are hypersensitive to Kratom, and may experience adverse reactions such as upset stomach or vomiting.