I have this script setup as a step within a frequently executed SQL Server Agent maintenance job. It prunes backup history older than 90 days. Just replace the 90 with how ever many days works for you:

[font="Courier New"]/*

Purges backup history from the MSDB database that is older than 90 days.

Use this to identify the oldest backup recorded in the MSDB database: select min(backup_finish_date) from backupset

*/

DECLARE @sql as nvarchar(max)

DECLARE @DatePurge as datetime

SET @DatePurge = CAST(GETDATE() - 90 AS datetime)

SET @sql = ''

Set @sql = '

use msdb

exec sp_delete_backuphistory ''' + LEFT(CONVERT(VARCHAR, @datepurge, 120), 10) + ''';'

exec (@sql)[/font]