When can I expect to be 'harvesting' leaves?

Any leaves that the plants drop in transport or shortly after receipt you can use for smoking as soon as you have dried them. There is no harm in trying anyway, even if the leaves are not full strength.

After a few weeks or so of growth you could try taking the odd one or two for drying and smoking whenever you wanted, obviously avoiding the very youngest and newly sprouting leaves. You should get some sort of experience even if the leaf is not fully mature.  When the plants get bigger you will then have more choice.  If you select more mature leaf then this will likely result in a stronger experience. 

It's also a good idea to pick any leaves that are looking perhaps past their best, maybe a bit mottled or drying at the edges.  The reason for this is that the leaves don't last forever. So an old leaf that is looking past its best will not get any better. In fact it will eventually wither and die and/or drop off after maybe turning pale and yellow first, so you may as well use it before it gets to this stage.

You will need to wait longer, a few months or so of growth, before you have enough leaves on the plant for regular chewing.  You'd have a fair sized plant after a couple of months growth, say, up to 60cm high, with ideal growing conditions and starting with a 25cm plant.  Check out the middle plant on the second picture on this linked page...>  

Note that when I say something like a few weeks or a few months growth, this does assume the plants are growing. If you receive plants in winter (i.e. in UK or in similar climate and you are not using artificial lights>) then they will grow, but much slower.  You will need to wait until the onset of springs before you see faster growth.