Howdy...I have made appx 1.2 Billion in the last 2 weeks and spent appx 8-10 hrs....
The best profit is to be made from Void Opals....you can find them in Void Opal hot spots after scanning Icy Rings, use Inara to check where the best sale price for VO's is and closest to your current location (appx sale price you should be looking for is 1.6Mil per tonne, also sack your NPC crew before yous sell as they will drain your profits, I found this the hard way!! Each core asteroid brings in appx 15-25 tonnes - so you'll get about 25-35 million credits per asteroid)
)
Once you have found and jumped into your hot-spot run your pulse wave analyser (try and buy a Type A as that gives you a good scan range) and look for very bright yellow and when up close they even show up green shaded asteroids, however there is only one asteroid shape currently that has a deep core for exploding it is "popcorn" shaped (ignore the small round ones, oval ones, or longish shaped ones).
Shoot your prospector limpet at the asteroid to show the fissures, shoot your seismic charges into the fissures with the correct explosive setting (low, med, or high as determined by the fissure once targeted), ignore the Bromelite cores they bring in next to nothing in profits.
Once the asteroid is blown up, use your collector limpets to bring in the free floating chunks, and once that is done use your Abrasion Blaster to blow off the remaining VO chunks off the Asteroid
(I dont use the mining lasers anymore nor the Subsurface Displacement missiles )
Edit Update: I forgot to mention to make sure to not overdo your explosion setting, watch the Seismic Charge Graph and ensure you get the Blue boxes filled...if you overdo the charge you will see red boxes appear in the graph - avoid that as that will decrease your mining chunk qty's, I also ensure to have good defences on my ship, when you first jump into a hot spot on all occasions I had at least one pirate spawn in as well....
As to number of limpets I generally start with 150 (my total cargo capacity is appx 480 tonnes), I also equipped two seismic missle launchers to give me more ammo (I set up diff fire groups for each one separately, so once the first one runs out of ammo I then swap to the next one, you can also fabricate the ammo but I dont bother

)