Hey Torque Effect,
The time limits vary for each test.
For Verbal Reasoning I got about 15-20 minutes for 30 questions. You get about 4-5 questions for each paragraph they make you read, but you don't really have time to read it again so make sure you read it properly the first time.
Numerical is probably the 2nd hardest test. You get about a minute per question, 20 or so total. You just need to focus and try not to spend too much time on a question, if it's taking too long, just make a guess and move on.
Calculation is the most time restrained one. It has different stages but luckily has a break in-between each one. It moves from giving you 11 seconds per question to just 6 seconds per question at the last stage, and has about 5 questions per stage.
The Checking test is also pretty stressful and you only get a few seconds to find the matching number or letter sequence. 25 questions total if I remember correctly.
And finally the Spatial Ability test is the one I found to be the hardest. You get less than a minute per question which is tough when you have to go through every one of the answer options and check them with the shown figure. Again, make sure you don't spend too much time on one question. It has 20-25 questions.
I'd say they are all somewhat difficult. If you did quite well in maths in high school, you should be alright though. I think most people would probably find the Calculation one to be the hardest since you have to calculate very fast.
However, before you start each test they tell you exactly how many questions there will be and how much time you have. They also give you practice questions that you can choose to do or skip. I strongly recommend doing them since some of the tests are different from the practice ones they have on the website. Calculation being the most different since the practice one on the website is like the numerical test with a total time limit, rather than a limit on each question
With the practice tests included, you're looking at about 3 hours total for the whole thing so make sure you have that much time available and no distractions before starting. You can't stop it and resume at a later time. It's all in one go. Also apparently there can be problems loading the questions if you use Internet Explorer. So I recommend using another browser like Chrome.
Good luck!