Most properties should be inspected once per year but there are no written rules on how often you should perform an inspection on your rental property. You could do one biannually or quarterly while keeping in mind your tenants may find this intrusive. Proper screening of your prospective tenant can help cut down on the amount of inspections you will need to do on the property and can help maintain a good relationship with your tenant.
If you do need to issue a 24 hour notice of inspection there are a few things you should keep in mind. In Ontario you are required to provide notice stating why you want to enter or inspect the property. You need to have the date of inspection and time you are planning to attend the property. You must also include the reason of entry plus any other activities that may occur while you are there, like taking pictures. Unless its an emergency, you are only allowed to enter the premises between 8AM and 8PM. Provided you have given proper notice, you can enter the unit whether the tenant is home or not.