I guess the answer depends on how much work you have to do. If you're starting from near "scratch", you may want to go for a year. If you already have a lot of material that's been collected by other family members, then you may want to go shorter.
Also remember that there are some good free sources of genealogical data on the Web. Even Ancestry.com hosts a lot of free stuff. FamilySearch.org has probably the 2nd best set of genealogical data on-line (and they keep adding more and more stuff)... and it's all free. Ancestry.com also hosts the RootsWeb WorldConnect database that contains info on 575 million names through the publication of family trees. RootsWeb also hosts surname and state/county genealogy forums... all free. They also host a lot of county genealogical society web sites that contain some good stuff. And lastly, non-subscribers can still search Ancestry.com for free... you just have to pay to see the detailed records. I've found "clues" doing free searches on Ancestry and then been able to look up the records using other sources.
