The scriptures do not relay every detail of every account, but only those that are considered to be necessary. As the scriptures are the word of God recorded through the agency of men, then the author is ultimately the Holy Spirit.
2 Pet. 1:21,
"for not by will of man did ever prophecy come, but by the Holy Spirit borne on holy men of God spake." (YLT)
Then as the ulitmate author of Ruth, the Holy Spirit deliberately did not provide the exact timeline. We can only speculate on His reasons for doing so, which most probably was that He did not consider it to be critical to the message of the book.
However, the people involved in the story of Ruth can offer more definition of the time line. Boaz was the son of Salmon and Rahab (Ruth 4:20; Matt. 1:5).
One Bible time line places the famine from the raids of Moab about 1294 BC between the 40 years of peace under Othniel and before the 80 years of peace under Ehud of Judges chap. 3. Source: here
Another source has determined that Boaz was about 50 years old when he met Ruth.
"The book of Ruth itself gives a very clear indication that the story begins when the judges ruled the nation of Israel. The LORD gave judges after the death of Joshua. Joshua died in 1399 BC at 110 years. If Boaz was born around 1428 BC (conquering Jericho), he must be around 30 years of age when Joshua died. Adding 10 years for Elimelech’s trip to Moab and Naomi’s return and a few years for that generation to die, we get around 1385 BC which places Boaz to be around 40 to 50 years of age.
The events in the book of Ruth happened around 1385 BC, nearly two decades after the death of Joshua when Boaz was about 40 to 50 years old and five decades after Israel fought against Moab before entering the promised land." Source: TrumpetCall.org
However, every bible time line seems to vary by about 50 years or so depending upon certain starting points, and other details, so we cannot be absolutely certain of the exact times. The time line at BibleHub places Ehud's rule at 1316 BC (Jud. 3:15) and the story of Naomi, Ruth, and Boaz at 1140 BC. Source: here
If we allow the variances for human error, and for gaps in the details the Holy Spirit did not provide, we can still pin point the time period between the judges of Othniel and Ehud.