Question on Buying Expired Domains

Hi Everyone,

It is a pleasure to be here. I have "lurked" here on an off for a while and have learned a lot!

This is a very knowledgeable group here and I have a question for anyone with experience / theories on buying expired domains for profit or to use as a tool for an existing website.

I have read recently that buying expired domains can be helpful in harnessing existing traffic and also gaining quality back links if the site had good page rank and you use it to link to an existing site. Of course it is better if the original site was relevant in content and domain title to your own site.

My question is, that if the site owner let it expire and there is no longer a web site there, how long can I expect to get a boost from that domains page rank?

If I put up a simple content site related to the domain and my existing website
I would not have the same page rank as the original more established site had.

How often is the site spidered and updated? If I never submit my new site / domain to the SEO's for spidering/indexing, will I continue to get the old page rank indefinitely?

Thanks,

Jay