How many minutes are in 100mb of a AVI format video?

.avi is a container format; the file extension itself provides no indication whatsoever of what codec is being used by the audio and video inside it. Because there are many variables, including codec, bit rate, the number of pictures used in the compression algorithm, the sampling size of the audio, etc..., it is impossible to predict with any real accuracy how long a video of an arbitrary file size is.