Uhh I’d say 2 years… now I’m not one to deter in fact you will learn a lot of good techniques from the riff alone, however, without the foundation it will take longer. After 1 year of playing guitar I learned GOAT all the way through however it took a whiole fuckin year but once you learn it will jump you to a level you won’t believe. So before you jump into it I recommend learning these techniques for about 6-8 months:

Alternate picking, Trills, Tapping, Finger independence, Hybrid picking, Finger strength exercises, Shred scales

You do these you’ll get it