A custom brush will save a ton of time, but should be used sparingly else you get that cloned look. You can mix it with some hand-painted "hero" grass. Some tutorials here: http://wiki.polycount.com/CategoryConceptEnvironment http://wiki.polycount.com/TexturingTutorials#Vig.27s_Collection_of_Bookmarked_Tutorials
"Hero" items are objects in the scene that get the most attention, so these typically have more detail than the rest. They are individually painted. The larger areas of medium-attention grass can be painted very quickly, and the rest is just a suggestion of grass (no individual blades). Saves time, and in the end it…
If you wanted to do something quick, make yourself a grass blade shaped photoshop brush and play with the settings. You'll probably want angle and size jitter, and colour dynamics. Once you've got some values in those areas, pick a couple of greens for your foreground nad background and paint. that should get you at least…