First thing to consider while designing toast is that it should contain few words or one line of text because it is going to visible on screen for some definite time and will disappear without any user interaction.
Snackbars should contain a single line of text directly related to the operation performed.
Google MD guidelines wrote above line for snacker but I believe it is equally important for toast also.
Try to keep in mind about average reading speed of human which may vary geographically.
According to this articlein dailymail.co.uk:
The average speed for adults is 300 words a minute
which means 5 words per second (Now, this looks challenging to me but people in UK might be comfortable with 5words/s, also reading novel or news article is different that a small message as user environment may be totally different). Mobiles are used in environment where people’s attention is split, so they will definitely take more than a second to read and interpret 5 words (If we consider UK's case). Also consider there will be people who take more time than average, they may read only 3 or four words per second. Your design should consider worst case. It’s not possible for me to provide any bottomline but based on your target audience you can run a small research to test your actual toast message.
If your are planning to use snack-bar which has some action to do (e.g. : undo action ) its better to keep it on screen until user interact with it or touch any other area on screen.