30 minutes of revision is actually about right. You should be doing between 25-30 minutes of revision and then having a 5 minute break before having another session of revision. Here are some links to posts about the issues you’ve mentioned:
- Here are some general tips for mathematics.
- A link to some algebra study guides and notes on Sparknotes.
- Some algebra study tips.
- How to learn algebra (wiki how).
- How to become better at algebra (wiki how)
- General study tips for Maths