More formal:
* How much?
* In what degree?
* To what degree?
* To what degree of accuracy?
* How significantly?
* To what level?
* How far?
* How deeply?
More informal:
* How much so?
* Just how?
* To what degree?
* How big a deal is this?
* How significant is this?
* How important is this?
The best alternative will depend on the specific context of your sentence.