How long does it take for liquid to boil in a crock pot/slow cooker?

I have a recipe for baked beans that asks me to boil water, then add it to the crock pot, then turn it on high until it boils, before switching it to low. This is supposed to cook 10-12 hours, so I planned for that amount of time, plus a little extra to let it boil, but after 45 minutes it had just barely started to bubble. I switched it on low anyway because after searching the internet I found either recipes with similar instructions or information that said that the crock pot isn't supposed to boil. Has anyone followed a similar recipe? How long does it take the crock pot to boil on high?