congratulations on completing 2800Km on you new ride.
I have also completed 2000km on my GS150 today.
Regarding your questions,
Engine Lubrication Oil: I changed it every 1000-1500Km interval, although the Suzuki Showroom mechanic told me that Oil can be used up to 3000Km. I however, prefer to keep the engine clean of burnt ashes. If for some reason oil can't be changed, I guess we should check its level at least and refill it up to the required level.
Engine Oil Filter: The mechanic says, first time it should be changed after 1000km maximum. Afterwards, we can safely replace the oil filter after every 3000km.
Air Filter: Should be washed whenever desired, usually I make sure to clean it (or get it cleaned) after every 1000Km or 2 weeks (keeping in view the dust conditions of Northern Lahore).
Spark Plug: If you keep your engine oil clean, you don't have to bother much about that. Yet, after every 3000km, have a look at the spark plug, if it has carbon deposit on its terminals, use sand paper to clean them. And adjust the gap between two terminals if required.
Wish you all the best with your wonderful ride.