All our commercial customers pay every 4 weeks if under about £10 per clean. It saves a lot of time.
So for price per 4 weeks:
1 £15
2 £15 or £20
3 £15 or £20
4 £20 or £25
Shops and commercial work is/are so quick and if you can get lots of them, they become a stable income week in week out. For instance, the first one you mentioned. I'd reckon on at most 5 minutes. Possibly less after a few cleans as you get to know the job.
It's worth remembering that by and large the standard doesn't need to be a high as for houses, and that the prices are usually lower when compared to resindentual work.
Shops only usually start to become really worth it when you build up a number of them. You'll usually find as time goes by you will get more work locally simply because you turn up every week. Also, all our commercial work is done during the day and not early in the morning because I want the customer to know for a fact that we turn up every week. I also have pre printed 'your windows have been cleaned' compliment slips which we date and leave on the counter.
We started with 6 hard won commercial jobs 4 years ago and now have 149, some weekly, some 2 weekly and some monthly - so it's worth sticking at it. From time to time you will get a really good earner.
Also, all you do every week or whatever is turn up and do the work. No phoning, little organisation and no access hassle. Simple and easy work.
If I had my way we'd only do commercial work.