Thanks @de_vries for providing great answers. I think you covered it all but I'll try to add some flavours to it.
Warm welcome @Alferic! Happy to see you here.
1) Yes, it typically only take minutes before you can see the photos on the map. It might take us a little longer to process them to nice transitions. It depends a bit on the load on the system.
2)a & c Take as many photos you like in a sequence. We try to emphasise to take at least around 50 if the street allows for it but if the street is shorter then there will be fewer photos. But general rule is always better with more than less photos. For capturing instructions pls check: http://www.mapillary.com/map/help?q=how-to-take-great-photos
2a, The app will automatically adjust this and take a photo every 4 sec when walking and 2 sec when driving.
2b, Yes, that is great. And you will find that information in our FAQ (same link as above) on the web.
Let us know if there is anything else!