Which Moodle version did you first install when you started using Moodle? If it was anything older than 2.3.or 2.2 then you need to plan a bit.
If your database contains any NTEXT column types then your upgrade could take a long time and you need to be careful. During the upgrade Moodle will convert those deprecated NTEXT columns to NVARCHAR(MAX). This could take a while, depending on how big your database is.
Use this to check if your database has any 'NTEXT' columns:
SELECT
table_name,
column_name,
data_type
FROM information_schema.columns
WHERE data_type = 'NTEXT';
If you do have any NTEXT columns it means your Moodle has been upgraded several times from a Moodle version older than 2.2. So make sure your IIS and PHP timeouts are very large so that you don't timeout half way through the upgrade. And first test the upgrade on a test machine.