I dont have a lot of monthly users on my site (currently only around 150 per month). How many users do you need to be able to get half-decent results from remarketing?
I can't recall the exact number you need but i would imagine closer to 5000 per month or higher would be your best bet. The way I look at it is; if you have relatively low number of visitors <1000 you need to check if they have high engagement on your website? Does the majority make a purchase? If not then its clear you would need a much higher visitor count before remarketing in order to make the remarketing worth it and targeting more potential customers.
As I remember, you need to have at least 1000 users to be able to create a new audience group and create remarketing campaigns, so you will need to wait for more time to collect them.