Some Subarus will have to have the head gasket replaced due to leaking either coolant or oil. If you observe the are where the head bolts onto the block you may see some residue. 
This is what to look for. See the oil accumulation? Make sure your repair facility is aware of this and keeps thier eyes open when your car comes in. If you happen to notice either oil or coolant leaking be sure to get your car in ASAP to be inspected by a qualified mechanic. Allowing this problem to go unchecked could result in the engine overheating and may cost extra for the repairs. The valve guides are succeptible to heat damage and the head casting itself can warp or crack A Subaru gasket where the leak occured 
Typically a head gasket repair can take up to a week or more. You MUST send the head to the machine shop for inspection and testing to check for cracks or valve leakage. This can involve a few days depending on how busy the machine shop is. It is imperitive that the repair be done correctly. We recently had to repair an incorrect head gasket done by another shop only 15,000 miles ago. We believe they skipped the machine shop part which is crucial to longevity. The total cost to the customer was higher than unnessesary because some of the valves were damaged. The repair should last 100,000 miles or more depending on how well you care for the vehicle. If you need help with your Subaru call us at (916) 924-8040 for advice or to have questions answered.