Time to paint over time
There are two possibilities here, with different approaches:


To keep on working the wet paint.
To continue working over dried paint.


Both are very common and often combined.


Keeping on painting while the paint is still wet won't impede your working process as much, because you don't have to heed certain restrictions that guarantee the quality of your painting - in principal, you can "just keep painting", with relatively long intervals.+
A possible disadvantage - depending on style, subject and goal - is that you might need more control over your brush and sufficient drawing skills, as it is easy to lose the vibrancy of colours by mixing them up too much as you try to correct mistakes (although you can always scrape off parts using a palette knife).  

The wet-in-wet or alla prima method was used by the impressionists (whose very name was derived from their (or initially Monet's) habit of 'quick' painting) and in general by people painting en plein air, which is still very common. Live portraits are often done this way, as having a model remain still for a while requires a quick and efficient technique. 
Working over (partially) dried paint is a bit more complicated, since - for the sake of the longevity of your artwork - you should heed the fat-over-lean rule, and never apply thinner paint layers over thicker ones, as this will cause surface tension resulting in craquelure or even chipping of the paint. This is already something to take into consideration when the paint has not cured yet.
Still, this is the most common technique - especially since most alla prima paintings are often retouched later on (days, months, years later) by the artist to make corrections, add detail, or even change the composition.
What's also important, especially if you're only starting to paint, is to not let this impede your process: even professional artists don't always care for the chemistry behind painting, so if you just want to experiment, or do exercises, or just have fun with it, knock yourself out. This is a medium, after all, and if your work unexpectedly ends up in a museum somewhere 150 years from now, there are restorators who will gladly earn their money treating your aberration ;)  




+ The drying period of oil paints depends on several factors: 


(Ambient) temperature;
(Ambient) humidity;
The amount of oil;
The amount and nature of additives (thinner, mediums);
The pigment;
The support; and
The thickness of the paint layer.


As you can imagine, based on those factors, drying times are naturally very relative.
In my experience, in averagely humid conditions (marine west coast climate) and with temperatures around 20° Celsius, most paints will remain manipulatable for at least two days (but be careful with earth pigments and blacks). The thicker the paint layer, the longer the drying process, so you have some say in the matter - glazings, for example, won't last that long.