What is a sensible user service schedule for a professional work bike in non loaded applications such as food delivery?
There is a similar question (but not the same question) here and here. 

The short version is that regular upkeep -- chain replacement, cassette replacement, lubrication, brakes (modulo say rebleeding a hydraulic set), tires &amp; tubes you should do at home as needed (and I've given some guidelines for the first two), and you should be familiar with basic adjustments for shifting and brakes. The shop should do repacking of the hubs (every 7500 - 10000 miles) and making sure the wheels are in good condition (this may be more often). Replace other things as they go. It'll likely be faster once you do it a time or two at home for the regular upkeep to keep doing it at home rather than taking it to a shop. You just need a few basic tools on hand (hex keys and a few special tools mentioned below). 

I'd suggest investing in a chain checker (e.g. Park Tool CC-3.2) to check chain wear. Expect a chain to last, say, 1500 miles (this is conservative if you ride in clean conditions; use the checker to see if its worn or not). Replace when worn. You should be able to replace the chain yourself faster than a shop since all you have to do is pull the old one off, size a new one (e.g. by using the old one as a guide, its just pushing out a pin) and put a new one on (usually done with a quick link). Oil your chain and things like derailleur pivots and pulleys weekly. 

You'll get a few chains per cassette/freewheel (maybe 2-3 of them). You should see this page on how to identify worn cassettes. Again, I'd suggest having a few cassettes/freewheels at home (depending on what your bike uses) and just changing them yourself. It's a quick job (less than 10 minutes) and just requires the appropriate freewheel/cassette tool, a big crescent wrench/vise and possibly a chain whip.  

Chainrings generally last a long time -- I'd expect 15000+ miles if you don't dent them up on road use. Again, check for wear or dents periodically. 

Brakes are important. If you're running rim brakes, make sure your rims have good braking surface (they last a very long time) and the pads are good and the brakes are adjusted. Again, wear depends on riding conditions and you can replace your rim brakes faster than taking it to a shop. You will also need to do adjustments if you have mechanical disc brakes (and replace pads periodically). Hydraulic brakes do need pads and occasional rebleeds/new fluid (if air gets into the system, for example; this should be on the order of year(s) though) but they do some amount of self adjustment. 

Learn to adjust your shifters and brakes as necesssary. 

At the mileage you're putting, I'd repack the hubs and have the wheels serviced in general (making sure they're true and what not) every 6-12 months (so lets say 9 months). I'd leave this to the shop. 

Replace tires and tubes as needed (so when you get flats/slightly before they start giving you flats). You'll likely be going through a set of tires maybe every 8-12 months. Again, something you can do quickly at home. 

Everything else, I'd say replace as they go (e.g. bottom bracket, saddle, pedals). If you want to be cautious, you could get a new set of cables yearly or something, but its not really necessary unless you live somewhere where the cables rust up and get ruined.

Finally, I'd suggest having a backup bike on hand if possible. Something a bit crappy, but will get the job done if your regular bike needs some upkeep (e.g. if for some reason theres a week turnaround on some repair, you have something to work with).