There seems to be two types: one that will come on suddenly (often in the middle of the night) and see you throwing up all night, feeling pretty shit the next day and that's it, and the other, much worse, will come on slower and last several days.
The difference is probably the strength of the poison, how well formed it was when ingested it and if viral or bacterial.
The bacteria responsible for diarrhoea and related symptoms normally die after 36 hours.
If it lasts longer than this, chances are you have nothing serious, but something treatable, for instance giardia (indicated by severe flatulence, stomach cramps and sulphurous belching) which is cured by Flagyl (Metronidazole) - see a pharmacist.
Staff in your hotel will normally help you find medical help or bring you water if on your own).
Some of the easiest places to get ill are where there are large numbers of tourists and the locals have adapted by offering western type food and/or a lot of food is saved, stored and re-heated. You might never have a problem in India, eating Indian food, but suffer in Nepal eating western style food.From a where you will stay to how you will get around.... The art of travel is about the most pretentious way of saying this. If you have done it before, all this will sound pretty boring and obvious, like someone telling you how to get up and go to college or work.A distinction should be drawn between general travellers' diarrhoea, and severe diarrhoea.The former which is more of an annoyance than a major problem, can normally be clocked up to changes in diet, time-zone, irregular eating and general stress.The latter is more serious and could be something much nastier.There are literary dozens of different strains of both and it's pointless to cover them here, but most encountered (normally bacterial) aren't too serious.Best advice: don't get paranoid about food poisoning otherwise it can cloud a trip and your experiences of some great food.Just allow time for it, and take the rough with the smooth, as it were - there is very little you can do about it once the poison is inside you apart from avoiding dehydration (Gatorade type sports drinks are much easier to drink than water and will replace a little energy/salt), get plenty of rest and let it come out of you (in whatever form - severe and continual vomiting that is preventing you from keeping water down and/or retching for long periods can be treated by tablets or an injection; either way consult a doctor if symptoms are persisting.It is about the simple things you may deal with on a day to day basis (especially if you are heading to poorer or developing countries).These are lessons you will learn quickly yourself and start to learn to get the best from and optimise to your advantage.