How much time should I plan to upgrade to SQL Server 2005 from SQL Server 2000?

It depends. The upgrade should be pretty straightforward, but there are a few gotchas, especially with regard to functionality that Microsoft has dropped starting in SQL Server 2005. Do some searches through your existing code for any of the following:

System Tables
System tables such as "systables", "sysindexes", "syscolumns", etc. are no longer available in SQL Server 2005, and need to be replaced by catalog view equivalents (see here)

Deprecated Functionality
Various system functions and stored procedures have been deprecated in SQL Server 2005. While some of the deprecated functionality may still work, an effort must be made to port whatever deprecated functionality is found to the new recommended equivalents for SQL Server 2005.

Use as a reference the following lists of deprecated functionality and recommended replacements:
http://msdn.microsoft.com/en-us/library/ms143729.aspx
http://msdn.microsoft.com/en-us/library/ms144262.aspx

Legacy Data Access Technologies
Microsoft no longer supports many of the older data-access technologies allowed in SQL Server 2000, notably:

• DB-Library
  
• Extended SQL (E-SQL)
  
• Data Access Objects (DAO)
  
• Remote Data Objects (RDO)

These may not work with SQL Server 2005, and would need to be replaced with an equivalent supported technology (e.g. ADO .NET).

Depending on how many instances of these you find throughout your code, you should buffer your project time accordingly.