How long is reasonable to ask for decision deadline extension?

[prokem]

I've been admitted to a dual degree program (my 1b choice), and the decision deadline for the two programs are April 15th and April 20th. I was waitlisted at my 1a choice, and the email said no one will be accepted from the wait list before April 15th, and more likely later than that. I would like to see if I can get in since it's my top choice and I'd like to make as informed a decision as possible, but I also don't want to leave my 1b choice in the dust. I am going to ask if I could have an extension on their decision deadlines, but how long would be reasonable to ask for? I'd really appreciate any insights that anyone can offer!

[AP]

A couple of days. However, when you write/call asking for an extension explain that you are waiting on other decisions on after April 15 and allow them to give you a timeframe, do not ask it yourself.