If you are poisoned, a message will be emoted by your character, depending on the severity of the poison. Additionally, your HP bar will become green. The severity of the poison, or poison strength can be deduced from the message that is emoted by your character:
0 - 100, "Bob looks ill."
100 - 200, "Bob looks extremely ill."
200 - 300, "Bob stumbles around in pain and confusion."
300 - 400, "Bob is wracked with extreme pain."
400 or more, "Bob begins to spasm uncontrollably."
In short, it depends on the poison itself. If you do something mildly wrong, like eating moldy or rotten food, the poison will merely drain your stamina and mana slowly. If you do something very wrong, like eat poisoned food, you could die, depending on what kind of poison was applied onto it.
Poison can have any combination and severity of these effects:
Health (Max Health) Decrease
Stamina (Max Stamina) Decrease
Mana (Max Mana) Decrease
Poison on Khaeros can last quite a bit, e.g. even 5 minutes. Poison becomes progressively more dangerous the longer you leave it uncured. At first, the damage or stat decrease might be minimal, but as the poison approaches the end of its duration, it will become much more serious.
Some poisons are faster than others. This is the so-called acting speed of the poison. In short, it tells you how often the poison applies its effects (damage, stat decrease, etc). To put it simply, the faster a poison acts, the more dangerous it is.