Spin Bike

Like Tavistock start with fairly short workouts and build up gradually. Almost as important is to check on your bike fit, there are lots of videos on the subject - have someone watch you while you pedal looking at leg extension, your position over the cranks etc it can make a huge difference to your comfort and efficiency.

Try to keep your cadence fairly high but in control (ie not bouncing around) and the ball of your foot on the pedal (you may even consider clipless pedals and shoes at some point in the future, with the stiff sole and correct foot position you have a more efficient pedal stroke)

I would recommend maybe 20-30 minute easy rides. You can get a workout in with a 15 minute though. The GCN link above is great. You can search Youtube for cycling for any time interval and get a bunch. I'll ride while watching TV shows sometimes, easy during the show and go harder during commercials.
Good luck, have fun!

Depending on your pace, 15-30 mins/session should be sufficient.

Does your spin bike show cadence and resistance?

To give you an idea, when I taught beginner spin this was one of my workouts, it was 3x a week for 15 minutes ride/5 minute stretches. You can drop back to a flat road any time you need to, just keep spinning without bouncing.

3 minutes at 20-25% resistance and a cadence of 75-85 to warm up.
2 minutes at 20-25% resistance and a cadence of 85-100. (Flat road)
2 minutes at 30-40% resistance and a cadence of 75-80. (Slight slope)
2 minutes back at 20-25% and a cadence of 90-100. (Flat road)
1 minute at 40-50% resistance and a cadence of 60-70. (Slow climb)
1 minute at 40-50% resistance and a cadence of 75-85. (Push to the top)
2 minutes back at 20-25% resistance and a cadence of 85-100. (Flat road)
2 minutes still at 20-25% resistance slow the cadence to 70-80.
Get off the bike and stretch all body parts head to toe.

This is easier when you have an instructor to call it, but you can do this or something similar at home with a timer.

If you don't have a display for cadence and resistance, 20-25% should feel like you could ride at a quick pace for quite a while without bouncing. Keeping your heart rate in an aerobic zone. You can still talk though a little breathy. If you are bouncing in the saddle add a bit more resistance rather than slowing your legs.

At 30-40% your legs will want to slow down, let them and ride steady state or you can push and hold the higher cadence for a little while. Your heart rate will raise, but still aerobic. Able to talk but labored.

At 50% & slow you are labored. At 50% and sprinting your heart rate should spike and you won't want to talk. A minute can be a long time. Heck 15 seconds can seem like a long time. Back off when you need to, but challenge yourself to eventually complete the sprint.

At home with no one to call it, try an easy interval. Change every 2 or 3 minutes from a flat road 20-30% resistance 85-100 cadence to a slight to moderate climb of 40% at 70-100 (depending on how frisky you feel). Repeat this for 10 to 15 minutes and add little variety by picking up your pace on your climb sections when you are ready.

As they said fit is very important.

This is also very important. Remember that it is your ride and your health. You do it for you. Make yourself proud. Each day is about progression not perfection. If you can't complete your planned ride, back of your tension and just spin easy for the rest of the time you planned. When you are done stretch and thank yourself for putting in the effort.

FYI: I did a very half a$$ed ride this morning. I gave blood yesterday, and wasn't really feeling it so I did more of a recovery ride than a "kill it" ride. I took a 30 minute class, I skipped half of the intervals and I rode the last 5 minutes flat instead of climbing. It was my time and my ride. I made it what I needed it to be even though the instructor was calling a different class. Nobody cares. Everybody has a different agenda and you make the workout suit you. That being said, if you never push and challenge your limits you will not progress into the awesome beast you are capable of becoming.

Best of luck,
Nikki

If you’re looking for guided workouts, I really like the CycleCast app. It’s $10/month, but the quality is high. It’s audio-only, but they’ve got lots of instructors and fun music and a variety of times and options. The peloton app is pretty good too. It’s got more variety (other exercises besides cycling), but is obviously more geared for the riders who have the peloton bike than app users.