When do I use 经常 and 时常? Can they be used interchangeably? If there is difference between the two words, what situation do I use each of them?
No. They are not interchangeable at all.
经常 bears more frequency than 时常. E.g. 我经常去看你 Vs. 我时常去看你. 我经常去看你 indicates I go see you more often than 我时常去看你.
If you say 我经常去打球，I'd think your frequency of playing ballgames is high. If you say 我时常去打球, I'd take that your frequency is normal/moderate (or probably a bit above normal), neither very high nor low.
both 经常 and 时常 means 'often'
经 = regularly
常 = frequently
经常 = often (if you do something regularly and frequently, you do it often)
时常 = often (in time)
You can use them interchangeably
经常遲到 = often late (emphasize the regularity of being late; being late is a regular behaviour for the subject)
时常遲到 = often late (emphasize the frequency of being late; the subject has been late many times)
经常不遵守规则 = often disobey the rule (disobey the rule is a regular behaviour for the subject)
时常不遵守规则 = often disobey the rule (the subject has disobeyed rule many times)
Another term for 'often' is '常常' (reduplication of the adjective 常). It also emphasize the frequency