How long would it take the mixer to mix these amounts?
20 bitcoins.
50 bitcoins.
100 bitcoins.
The way you ask the question suggests that you have to understand how a mixer works first.
A simple mixer works like this:
You send your coins (amount is irrelevant as long as the mixing service can cover it) to address A (belongs to the mixing service) and get your coins back from address B (which also belongs to the mixing service) .
When B is empty or has not enough to cover your input in A it would not be accepted before all coins from A have been moved to B.
So as long as B has sufficient coins the mixing takes as long as the mixing provider sees fit. Maybe they want 6 confirmations or maybe only 1, but the amount you want to mix makes no differnce besides for the security needs of the mixing service.
Less than 25 BTC - 1 confirmation;
Less than 250 BTC - 3 confirmations;
Less than 1000 BTC - 4 confirmations;
More than 1000 BTC - 5 confirmations.
https://bitmixer.io/faqs.html
