Translation:I reckon that it is time to go to the sauna.
I wrote "I suppose that it is time to go to the sauna." Which should have been accepted, but it was not. By the way, most people in northern and western US would most likely not use the word "reckon" as a thinking word. Use of the word "reckon" in most English dialects is not about simply thinking about something. It is about making a calculation. "The debts were reckoned" or, "He will get his reckoning" would both be correct uses of the word. The use of reckon in the sentence above would be very informal and even then only used in certain regions.
What you're referring to is called a zero relativiser, and that isn't a thing in Finnish. There may be situations where it may seem otherwise, but those are situations where Finnish doesn't need even an unspoken relativiser, for instance in the translation of "I think (that)...", which would be "minusta..." or "minun mielestä(ni)...". Possessive suffixes, like the one I just put in parenthesis, are optional as long as the possessive pronoun in the same phrase hasn't been omitted.