How can I tell if it's the right time to continue with my app, or should I just work at my full time job for some time?
2. Do some quick math: how many copies will you need to sell to cover your living expenses? If this is a mobile app you sell for $5, it's going to be a huge number.
3. How are you at marketing? If you have no experience at marketing you're basically relying on luck. And this is small business marketing, not "I'm a giant company doing branding" marketing, which is all you're likely to learn in school.
If you're just graduating you probably have few responsibilities. So find a job that isn't all-consuming, learn marketing and work on your app in your spare time.
You can discard 100% of what your friends and family say - they are not in the business of crushing your dreams.
Release your app, install basic analytics so that you know how many people are using it. Put a price on it to see if people are willing to pay for it.
Full time job is not an impediment to working on your app. Yes, it's going to be harder, but I wrote lots of code and shipped a couple of apps while working full time.
As to when to quit - it mostly depends on your level of optimism and willingness to take risks.
Personally, I would wait until I have a steadily growing number of users for the app.
Before leaving my last job, I was pulling all nighters working on a side project. I'd show up to work super sleepy and take naps in the meeting room. I put in notice after about week, realizing this wasn't fair to my employer, since I had no intention of slowing down with my app.
Get some work experience and save some money. Build it on the side (if you can/allowed per company policies/contract).
Does your new job allow you to work on side projects? If you work on a project on your own time and own hardware do you own it and the IP. Check your contract/signed documents.
Good luck.