Every country has its own copyright laws. In the US, the first thing it depends on is the Department of Justice, which decides whether to prosecute. This is the prosecution policy of the DoJ (having said that, there are also state laws against copyright infringement, so it is possible that some state attorney general will pursue a case... you need to look into the matter on a state-by-state basis: usually, the matter has to be prosecuted at the federal level). The relevant US law is here. One of three things must be true, for criminal prosecution. One is that the action was "for purposes of commercial advantage or private financial gain"; or,
"by the reproduction or distribution, including by electronic means, during any 180–day period, of 1 or more copies or phonorecords of 1 or more copyrighted works, which have a total retail value of more than $1,000"; or "by the distribution of a work being prepared for commercial distribution, by making it available on a computer network accessible to members of the public, if such person knew or should have known that the work was intended for commercial distribution".
The stipulation that this is "for personal use" precludes re-distribution cases, but leaves open the possibility of jail time for "massive" personal infringement. It is highly likely that many infringers do in fact download more than $1,000 worth of content in a 180 day period. This article addresses the "will you go to jail" question. The author notes that it is "highly unlikely", and "there were fewer than two hundred criminal intellectual property convictions in 2010" which included trademark cases... also, not every conviction results in imprisonment (fines are also possible). This report from the Bureau of Justice Statistics notes that
Of the 134 IP theft defendants sentenced in 2002, 58 (43%) received
some imprisonment with a term ranging from 1 to 46 months and a median
of 15 months. In 1998, 34% of defendants were sentenced to prison with
a range of 1 to 30 months and median of 10 months.
However, this does not narrowly report just "personal use infringement" imprisonments. One can speculate as to why the federal government is not more aggressive with prosecuting personal-use downloaders. The first reason (in the long list of reasons) is that personal-use downloaders are least likely to get caught. The last reason (on my list) is that under sentencing guidelines, a personal-use infringer is least likely to trigger imprisonment. This manual explains sentencing guidelines and the 43 levels of seriousness, and this explains the factors that go into computing the seriousness of infringement. Imprisonment is not statutorily mandated, so it is very unlikely that a level 8 infringement conviction will result in imprisonment.