I respectfully disagree somewhat with Skippyak. You should be having fun. If you're not, you're less likely to keep doing it. So if knackered, sweaty and out of gas makes you think you really don't like cycling so much after all, then don't go there, at least not now.
Not knowing anything about your age and general health, I would say that -- as long as your doctor says you're healthy enough for exercise -- you should go ahead and ride as much and as long as you like. If 30 minutes is leaving you exhausted, I would increase slowly. If you feel like you could keep going after 30 minutes, then by all means, keep going! Jump up to 40 or 45 minutes today or tomorrow and see how it goes. If you run out of gas before you're done, you can always stop and rest for a few minutes. Make sure to bring a water bottle, especially if you live in a hot climate.
The important thing is to listen to your body, and don't ignore pain that might indicate an injury (vs muscles that are sore because they've been working). The other important thing is to have fun! If you're enjoying yourself, you'll keep doing it, and soon you'll be riding so much that you'll be amazed that you thought a half-hour ride was a lot.
Good luck!!
- Gray 2010 carbon WSD road bike, Rivet Independence saddle
- Red hardtail 26" aluminum mountain bike, Bontrager Evoke WSD saddle
- Royal blue 2018 aluminum gravel bike, Rivet Pearl saddle
Gone but not forgotten:
- Silver 2003 aluminum road bike
- Two awesome worn out Juliana saddles