Air filter should be cleaned after EVERY Trial if this is done then one filter will last a lot longer before it needs replaced, the filter is one of the most important parts for the engine to run as it should. Every nut and bolt should be checked after each trial, once you get to know your bike you will find out which bolts come loose and the ones that never come loose.
Oil should be changed:-
Two Strokes Gearbox:- Synthetic Quarterly (can be left for 6 months)
Mineral Oil monthly
Four Strokes Crankcase:-Synthetic Quarterly (can be left for 6 months)
Mineral Oil Every Trial
Some of the riders change the oil after every trial regardless, this is over the top with modern day oils as there are no bad oils out there. (look at the oil like whisky, there are no bad whiskys out there just some better than others)
Plugs generally need to be looked at (standard Plug) every second trial(platinum Plug) these will last a lot longer and do not need the attention of a standard plug 6 months. The gap varies for different engines but the standard setting is .025" or .06mm (thickness of a hacksaw blade) most bikes will run at this setting. The type of oil also determines the condition of the plug mineral oil burns and forms carbon deposits, synthetic oil does not burn so no carbon deposits.
There are no points in a modern day trials bike, they are all electronic ignition with a Stator (stationary part inside the fly wheel) and CDI (Capacitor Discharge Ignition Box) in an Ideal world there should be no problems with these parts but they do fail and can cost up to £300.
Chain and Sprockets:-
These should be checked (especially the rear sprocket securing bolts) adjusted (1/2" or a Thumb width betwen the tensioner and the swinging arm) and oiled after every trial. The best method for lubrication is the spray on Q8 chain spray, Putoline Chain Lube, Castrol Chain Spray to name a few, most of them will work well. There are other methods like chain wax but you have to take the chain off and boil it in the wax (pain in the bum). If you stick to a strict regime then the chain and sprockets will last for quite a while, if you fail to maintan your bike then it will be replacing them every 8-12 weeks at an average £60.00 a time
The front tyre will last a lot longer than the rear as the rear does all the hard work, depending on how hard you are on the bike you should get a season out of the back tyre and possibly two seasons (unless you hit a rock and damage the tyre which will happen). Cost of replacing the tyre depends on what you prefer to ride with (I prefer Michelin) Cost of replacing tyre range from front £45.00 to £65.00 and rear £75.00 to £110.
Tyre Pressure should be run at 10lb to 6lb in the front and 8lb to 4lb in the rear tyre depending on the tyre choice, (Michelin tend to run softer and IRC 2lb to 4lb) and the weather conditions, dry hard conditions-harder tyre and muddy conditions-softer tyre.