'There really isn’t an average. It depends on too many factors.
The size of the plant. A single marijuana plant can be anywhere from two feet tall to over twenty feet tall, and even the more typical cultivated range is huge - the potential yield difference between a three-foot plant and a six-foot plant is enormous.
The strain you’re growing. Some naturally produce heavier buds or larger amounts of buds than others, some tend to branch more than others, some have closer node spacing than others - all these factors and more influence yield.
Whether you’re growing indoors or in a greenhouse, where there are limits on how big the plants can be allowed to grow both in height and diameter, or outdoors, where any such limits are likely to be much less restrictive, and may not exist at all.
What the growing conditions are. Marijuana will yield better in ideal conditions than in marginal ones (as well as producing higher quality results.)
How you train and prune the plant. A properly groomed and trained plant will yield more than one that was left entirely to its own devices… but that’s still better than overpruning and training too harshly, which will significantly reduce yield. (For example, some people think it’s a good idea to remove every single fan leaf from their plants. This is not, in fact, a good idea. They are harming their yields and quality by doing this. Those leaves are expensive for the plant to grow and are important sugar factories. You want to remove only those leaves which are shading buds or are otherwise poorly placed. Giving the plant a trim is good. Totally stripping her of fan leaves is bad.)
Any outbreaks of pests, disease, or nutrient problems (whether shortages or overages) will reduce your final yield as the plant must divert energy to deal with these problems. (That said, this is mostly true when this happens during bloom. If the plant is still in veg, it usually has time to recover from the problem before bloom begins, though if the problem was severe enough the plant may be permanently damaged and yield thus reduced.)
If you’re a skilled grower working with a good-yielding strain in an outdoor situation with close to ideal conditions, you can pull multiple pounds off a single six-foot plant. I got almost a pound off of one four-and-a-half foot plant in the 2017 growing season. On the other hand, if the genetics are bunk or your skills aren’t up to snuff or the conditions suck (or all of the above), you’ll be lucky to get an ounce off a two-footer.'