Execution Plan takes > 50 secs to show

I am trouble shooting a long running query and usually it won't take long to get an estimated execution plan, but when I click the 'Display Estimate execution plan' button on the SQL mgmt tool, it takes over 50 secs.. I ran into a lot of complex long running queries in the past but their execution plans are usually very fast to get < 5 secs. Does anyone know why sometimes it would take a long time to get an estimated execution plan?
W

It is not that complex. I saw others even more complex than this one. I observed the same behavior on a different computer. When I tried to trouble shoot the problem the first time and got the estimated execution plan instantly but after that, it takes forever. We are using SQL 2008 standard on a Windows 2008 enterprise x64 box.

Execute your query and then execute this query on another query analyzer is
select st.text,sp.* from sys.sysprocesses sp cross apply sys.dm_exec_sql_text(sp.sql_handle) sp order by sp.cpu desc
then checl what is the bottleneck,this query will help you

It depends on the Data also,in the past the data was low and in the present data is high ,that might be an issue of time taking