We recommend to place a link in the first 135 characters. This allows the entire link (25 characters) and message to be contained in the 160 character limit of the the SMS. 

If you insert the link at say character 150, this will cause a 2nd message to be sent. The link will span both messages. For older (not so smart phones) they might not join the message making the link non-clickable. 

Modern smartphones will join the messages making this a non-issue, it's just for older phones and we want to make sure everyone can click.