Like most other perennials, ornamental grasses typically take about 3-5 years in the ground before the clump is considered mature. After that time, you may want to consider dividing the clump if the interior portion appears to be dying out. And yes, with most OG's the clumps gradually increase on their own, although there are some grasses that spread quite a bit more vigorously via rhizomes.

Both of the species you refer to are clumpers and should be expected to follow that rough estimation. It DOES depend on the age of the plant at planting as well - here OG's can be found as large as 7G pots, especially the larger growing ones like some miscanthus and the pampas grasses. These are already pretty well mature :-)

Thanks for the help, gardengal. I guess I can count on my little project looking sort of piddly for a while until everything grows in.

I don't think they look piddly at all. If they're happy where you put them, they seem to grow much faster than you expect. A 4" pot grass will seemingly explode in its first season, and looks great while it's at it. I usually start mine in nursery beds for a season or two before moving them out into display areas of the garden.

Wilson, in my experience, a gallon pot of grass might look small at first, but they will mature quickly into a large clump within a year or two. If you are able to purchase 2 gallon pots or larger, then you will get a mature sized clump of grass the first year you plant them.

Even the grasses I've started from seed will usually bloom a little the 2nd year, and become good-sized clumps by the 3rd year.