You can do 1000mg for 18 weeks straight, but I wouldn't run that high of a dose for that long. It's just really unhealthy. I've gone up to 40 weeks without coming off, but I had 'cruise' periods where I let my body recover some.
Maybe do this instead:
Week 1-10: 1000mg Test
Week 11-15: 300mg Test
Week 16-25: 1000mg Test
That's 20 weeks at 1000mg, but spread between 25 weeks, so your body has time to recover a bit.
And if you were really wanting to make some changes:
Week 1-10: (week 1-4) 50mg Dbol / 750mg Test / 750mg EQ / 200mg Masteron (Bulking)
Week 11-15: 250mg Test
Week 16-25: 250mg Test / (weeks 19-25) 225mg Tren / 200mg Masteron / (weeks 21-25) 50mg Winny (Cutting)
Basically make the first 10 weeks adding pure mass, then weeks 11-25 (15 weeks) you can focus on diet and just get peeled and all granite like.
If you are going to do a competition, this would be a great plan, though doses would have to be massaged as you go along.