1 year regular preparation for jee mains is enough but only if you will stdy on regular basis and i can bet that if you are really a hard worker then you can score even more than 250 but only thing is practice the subject matters regularly and revise it. Self study is the best study in this world and as you are hardworking student so surely you are able to grab the things easily .

Remember this that success is 90percent hardwork and rest 10percent include other things . So I hope this will help you.

All the best start doing it now.