Marketing Director @ HubSpot & co-founder of Traffic Think Tank.
The best way to work around something like this is to map out the long-term strategy in phases. Build out a brief project map that outlines what they will receive within the 1-3 month period, the 4-7 month and the 8-12 month period. Set micro objectives for each period and this will give the client a bit more confidence in the short-term plans as well as the long.
The key thing to remember here is that the client will often be worried about being tied into a contract that doesn't deliver results. As a result, you need to show why you need the time that you do. One thing that I often throw in is an extra incentive for longer contract lengths - for example, an extra PR/content campaign or some paid advertising extras.
Try to assure them of some shorter term results that you can obtain as 'quick wins' and build their confidence this way - the major targets will always be longer term but if you can demonstrate that there will be progress between then they will be a lot more receptive.