This is still fairly low dosage. It isn't antimanic/antipsychotic dosage – that begins at roughly 300 mg. So the question is, why are you taking Seroquel and why are you titrating up? What is your diagnosis? What are your symptoms? What was insufficient in the relief you were getting at 75 mg that prompted your prescriber to up you to 150 mg? When you ask "when is supposed to work?", it would help to clarify what effect you are expecting to see.

Also, is it ER or IR (extended release or instant release)? When do you take your dose? At bedtime?

I take 75 mg of Seroquel for sleep. Lots of people take it off label at 75 mg or less for sleep, including people without any mental health diagnosis. This is a very common pattern of off label prescribing for Seroquel. But to stabilize moods, you need significantly more.