Your question is pretty obvious being a 12th class board student who has the burden to finish their syllabus before the board exam. See, there is not a fixed time to study maths in class 12th, but giving 2 hours every day is more than enough.

If you have a well organised time table that allows you to study for 6-8 hours per day, then I would recommend you to give 2 hours to Maths. However, the time you should spend on your study can be determined in 2 ways:

1. By Assessing your strength in Maths Concepts
2. By Knowing the Board Exam Time table

1. By Assessing your strength in Maths Concepts

If your basics are very clear in maths then you can even reduce the recommended time: 2 hr/day. Because in that case, you will only need to solve the given exercises, rather than revising the same concepts time and again.

If this is not the case and you think you will have to pay special attention to the maths basic concepts then you will be required to spend a lot more than 2 hours per day. 

Therefore, before you decide how much of your time should be given to maths, make sure you have assessed your basic knowledge by solving easy level questions.

2. By Knowing the Board Exam Time table

This is not rare, but the majority of students start becoming sincere and conscious for their study time when their board exams dates are near. So, before giving a time frame to study maths you must know how many times are left for your final exams.

In case the board exams are about 8 and 9 months later then 2 hours per day is enough to finish the CBSE Class 12 Syllabus. But if your board exam has 2-3 months left then you have to give more time to study based on your level of proficiency in the math topics. 

In the last minute preparation you can even use the Class 12 Maths NCERT Solutions to boost your learning pace.