Help Me Calculate Concentrated Grape Juice

A saison beer recipe I'm brewing calls for 48oz of Alexander's Sun Country Merlot concentrate but I've decided to make the concentrate myself. I bought 5 gallons of Zinfindel grape juice from a winemaking supply store and I plan to boil the grape juice to concentrate it and then can it for later use throughout the year.

Sun Country's Merlot Concentrate is 68 brix. Any idea how long I have to boil the grape juice (assume 15% evaporation) to reach 68 brix (or ~1.3 SG)? I don't have a hydrometer that measures that high and I'm hoping to boil the grape juice tonight whle I'm brewing the saison.

Any help is appreciated.

I can't imagine that boiled grape juice would be any good...

Would you mind explaining a bit further? My LHBS didn't have the merlot concentrate I was looking for so they recommended I buy juice and boil it down to concentrate it.

Boiled juice would taste like, well boiled juice. I wouldn't take some nice juice and concentrate it like that! If I had decent juice, I'd just freeze it as is and use it. Boiling fruit changes the flavor of it- think of the difference between a fresh apple and the apple in an apple pie. Or in fresh strawberries vs. canned strawberries. A "cooked fruit" flavor is unpleasant in most wines, I'd think.

However, I do boil bananas for my banana wine!

Thanks for the response so far. At this point I'm thinking I should bottle the juice in santized mason jars and freeze it for later use. Any idea how I can take this grape juice from 22 brix to 68?