Don't interrupt me every time.
As a return user, I'd get especially frustrated. After several log-ins, I'd learn the site and know generally where I need to go in order to accomplish my goals. Additionally, I'd learn that I have one of two options:
Wait for ~3-5 seconds for the modal to interrupt me so that I can immediately dismiss it and move on with my business, or
Try and race the invisible timer so I can get away from the modal before it jumps in my face.
Either one of these is going to get pretty annoying pretty fast.
If you really, really need to, consider an onboarding tutorial.
If you're worried a user won't be able to figure out your app (perhaps its sufficiently complex by nature), then consider implementing an onboarding tutorial that guides them through a few of the most important features. Show them where to find their account balance (which seems possibly useful to always show), and other critical information and core features.
Key points to remember for an onboarding tutorial:
- Keep it short. A walkthrough of every single feature will likely do more to overwhelm the user and discourage them from learning your app than it will to help them use it.
- Allow them to skip/exit at any time, and resume/restart at any time. It's quite possible they already are familiar with your site or app (perhaps they know someone that has used it) and don't need the tutorial, or maybe they want to try to figure it out on their own first. In either case, tell them where to find the walkthrough later, should they decide they need it.
- Let them know how to find more in-depth help. The tutorial should help the user get oriented and acquainted with your site/app. If you have an FAQ or Help section, point the user there for further inquiries.