It depends. Every liquid or dry wheat extract has a percentage of barley extract in there. I don't think any 100% wheat extracts exist for the similar reasons there's no beer that's made from 100% wheat malt.
For some it's as low as 50/50 wheat/barley, some are 75/25. Briess is 65%/35% and pretty standard for American extract brewers, so I'd stick to that ratio as it's pretty safe. So for example if a recipe calls for 10lbs of wheat malt extract, you'd be fine using 6.5 pounds of wheat malt and 3.5 pounds of barley malt, 5lbs of wheat malt extract would be 3.25/1.75, etc.