Sweet Spot Intervals - How many minutes a week?

There is no hard and fast rule. Also, what are you seeking to improve? As you are doing SS, I am guessing you want to get a bit fitter and a bit faster?

What you are doing sounds reasonable and if you do a solid block of it, 3-4 months perhaps, you will get fitter.

I always bang on about consistency and training over a long period to see real improvements in fitness and performance.

People respond to training differently so this is just an example but from personal experience my aerobic fitness was rubbish when I started taking cycling seriously years back (anaerobic was good and compensated for how poor I was anaerobically). The sports and exercise physiologist I did testing with gave me a plan which involved a lot of tempo (basically SS) rides in 3-4 month blocks. This was around 2 or 3 rides per week of between 60-90 minutes in duration. These were full on rides, not intervals.

This made a dramatic improvement to my aerobic capacity but over an 18-24 month period. It al depends on what you want to improve and how committed you are to doing so.

To put it into perspective, power wise, this saw my FTP go up around 30% over a two year period. Those rides went from an average speed of 28/29kph to 34/35kph (roads and conditions were consistent so I think Avg speed is reasonable to use). Like I said, this is personal but it gives an idea of what can be achieved.

What you are doing sounds fine for that, you should expect to see some improvements.

Once you get to a point where the intervals are comfortable, maybe even a bit easy I would start to try the longer temp rides outside. Start off with 46-60 mins and then graduate up slowly. If you are doing them at the correct effort then eventually 2-3hr rides at this intensity are feasible but hard.

Aerobic fitness and having a solid base of this is the thing that will get your power and speed up. It does require a fair bit of training over an extended period to do it though.

As has been said above start at say 3x8min but I build to 3x20min with 5min recovery between over 4 to 6 weeks. Then redo your FTP test and reset the SS zones.

One SS session a week is OK for me but a second session may be hills or shorter/ more intense reps

bobmcstuff said:

If you look at e.g., the TrainerRoad plans (which happens to be what I have the most experience with) you would start off with shorter intervals and then start to increase the length of them and bring them together (so you are doing longer intervals) as you progress - if you keep doing exactly the same workouts you will eventually stagnate.

An example from their mid-volume sweet spot base plan has 6 intervals totalling 36 minutes sweetspot on the Thursday of week 2 then in week 3 you have a similar workout with slightly longer intervals for 42 minutes total sweetspot. By the last week, you have 4x12 minute intervals.

So you might build from 3x8, to 3x10, then 2x15 and so on.

Here's the TR plan I was referring to if you are interested: https://www.trainerroad.com/app/cycling/plans/147-sweet-spot-base-mid-volume-i

I've had good results with Trainer Road sweetspot plans in the past and have just started back on one recently after a lack of riding lately.
It's also worth listening to the Trainer Road podcasts, they have some pretty good information on them about different training levels and how much to do etc.