Certified GKH Gardening Expert
Ideally you should mulch as far as the drip line - this is the outer reach of the plant's branches. As a rule, roots tend to grow out to the drip line so they can get to the water that drips down the outside of the plant, just like the name suggests. You're mulching to keep the roots moist and temperature regulated, so you want to cover their entire area.
Just make sure to leave a few inches between the mulch and the crown of the plant in order to prevent disease and pests.
You can read more about forsythia care here: https://www.gardeningknowhow.com/ornamental/shrubs/forsythia/forsythia-shrub-care.htm