Are you able to confirm what type of wood the gates are made from please? This will help with the advice we can provide.
Knowing how long the oil finish will last is dependent on how much wind, rain and sun the gates are exposed to. Because of this, Osmo are unable to give a specific time frame of how long the oil finish will last but as a very rough guide from other users experiences, the finish will probably be good for 2 plus years, before requiring a maintenance coat. This is just an estimation and is very dependent on the amount of weathering they receive.
Before oiling the gates we recommend that they are treated with a mould and mildew cleaner. This includes all edges, top and bottom, and especially any 'cut ends' or 'end grain'. Although there may not be any visible signs of mould or algae when the gates are delivered or installed, If the wood has at anytime been damp or wet, mould spores could already be present in the surface grain of the timber. Using a mould and mildew cleaner will kill off any spores in the wood prior to oiling.
The next phase would be to treat the wood with a wood preservative. Using a wood preserver will help to protect the wood from mould, algae and insect attack. Osmo WR Base Coat (4001) is a good choice and compatible with Osmo UV Protection Oil Extra.
It's worth checking the gates on a semi-regular basis for signs of mould, algae and water penetration. The first places to be affected are usually the flat, horizontal surfaces where water can collect and sit. This can degrade the finish and promote organic growth over-time. If there is any sign that these surfaces have lost there ability to repel water, simply apply a thin maintenance coat of oil when the wood is dry. In the event of mould or algae appearing, re-treat with the mould and mildew cleaner then re-oil the affected area once dry
In general, hardwood gates made from exotic hardwoods such as teak and Iroko are naturally more resistant to weathering, biological threats and rot. Gates made from Oak or softwoods need more care and maintenance but if well maintained, can last for many decades.