how long might I work as a flight instructor to build 1000hr fly for a regional?
You might work 2 years to build 1000 hours, or you might work 10 years to build 1000 hours. That depends on you, and your ability to get and keep students.
How long before you can fly for a regional?
That could be 2 years, or it could be never.
1000 hours does not mean you can fly for a regional.
It means you have 1000 hours, that's all.