A solo 401k owner does not have unlimited time to complete a rollover. Instead, there is a 60-day window to to accomplish it. The 60-day clock starts when the distribution is received by the solo 401k owner During the 60-day period the funds may be used by the solo 401k owner for any purpose.
While there are some very limited exceptions, in general, if the deadline is missed then no rollover is possible. The distribution would be taxable and subject to penalty, if applicable. To keep this from occurring, complete the rollover before the 60-day rollover window expires.
IRA-to-IRA or Roth-to-Roth rollovers are subject to a once-per-year rule. For purposes of this rule, traditional and Roth IRAs are aggregated. This means that a distribution and subsequent rollover between your Roth IRAs will prevent another rollover within a one-year period between either your traditional IRAs or other Roth IRAs.
The 60-day rollover rule limits the IRA owner to only one rollover of IRA funds every 12 months. Specifically, The 12-month period is a full 12 months. For example, if you receive your IRA distribution in November, you will not be eligible to do another 60-day rollover from any IRA or Roth IRA until the following November. The 12 months clock start on the date the IRA funds are received by the IRA owner.
Because a solo 401k plan is a company plan, rollovers from a solo 401k plan to an IRA are not subject to the once-per year rollover rule. Roth IRA and Roth Solo 401k conversions are not subject to the 60-day rollover rule either.
How can 60-day rollover problems be prevented? Solo 401k owners should check their IRAs to identify if any rollovers have been processed in the past 12 months before processing any IRA rollovers to a solo 401k plan. Even better, make sure that when moving funds from an IRA to a solo 401k that the movement of those funds are processed as an IRA direct-rollover (i.e., the check is made payable in the name of the solo 401k plan) because IRA direct rollovers are not subject to the 60-day rollover rule and thus unlimited number can be processed during a 12 month period.