Huub has your answers. I'll elaborate a bit on other aspects that you may find helpful.
First, toning by time is inaccurate. That's because toner (especially at weaker dilutions like 1+9 or higher) becomes weaker and loses activity with each print toned. Print number 20 from the same toner batch will not tone like print number 1. Second, every subject tones a bit differently due the different distribution of tones in different prints. One print might look great at 10 minutes and another not. I prefer to tone visually. I keep a wet, untoned print next to the toner tray for comparison (usually slapped on the bottom of a tray leaning upright behind the toner tray). I tone under light that I'd consider ideal for display; for me, rather bright incandescent light with a touch of filtered sunlight. Pull the print just before it reaches the desired tone change and rinse. Then keep this one and the untoned print around for comparison as you tone other prints. If you're toning just a few prints, the same time may work just fine, but if you're toning 5 or more of the same print, expect to tone a bit longer for the last ones.
If you haven't seen my posts on replenishing and reusing selenium toner so you never have to discard it (and selenium into the environment), do a search on my name here and over at
www.largeformatphotography.info/forum/ . Short version: filter your toner solution before and after toning with coffee filters or filter paper. When toning times become too long, add a bit of concentrate from the bottle. Adjust your dilution to bring toning times into a manageable range; I like 4-6 minutes. Too short toning times can result in uneven toning.
If you use different papers, be aware that papers tone at markedly different rates. I have two gallon jugs of toner, one marked "weak" and one marked "strong." I keep notes so I know which strength I need for various papers. I'm not sure of the exact dilution of these, since I adjust dilution to get a desired toning time.
Shuffling many prints in a toning tray works just fine, but if your toning is progressing rather rapidly, it's good to keep track of which print went in first and take them out in order and in the interval you submerged them. The number of prints you can tone at once depends on print size and your dexterity.
Consider your toner to be the same as fixer; wash fully after toning. Use a wash aid after the toning step if toning fiber base prints. You may see direction to mix a wash aid directly with the toner. This is an old and discredited practice that results in wasted toner. Keep the steps separate.
And, be sure and wear nitrile gloves when immersing your hands in toner!
Best,
Doremus