There is no evidence that long-term creatine supplementation in healthy people is harmful. In fact, there have been studies lasting over 5 years of continuous use that have yielded no negative effects. Just make sure to go off 3-5 days before a blood test so that you don’t get a spurious reading. As far as usage length of time for competitive athletes , we recommend cycling it throughout the year during intense training cycles (see below and respective pages in the document from the link below).
All the information you would possibly need safety regarding creatine is found here. You’ll find the safety and blood test info on pages 13 and 14.
Here’s a recommendation summary for use:
Dosing generally starts with a loading phase of 20 g of creatine monohydrate per day (CM)/d or 0.14 g/lb split into four daily intakes of 5 g each, followed by a maintenance phase of 3-5 g CM/d or 0.014 g/lb/d (dotFIT prefers .04 g lb/d based on experience) for the duration of the supplementation period. The length of the supplementation period would be based on the goal, but generally last 12-16 weeks and cycled throughout the year for competitive athletes