The IRS sets limits on an annual basis as to what contribution may be made to your Self Directed IRA and/or your Solo 401(k).
IRA’s: If you are under age 50 you may contribute up to the maximum of $5,500, if age 50 or over, you may contribute up to $6,500. As long as you do not exceed your eligible limit you may contribute as often as you’d like during the year (monthly, quarterly, bi-annually, etc)
For Simplified Employee Pension IRA (SEP), the annual contribution is $54,000
For Solo 401k’s for 2018 the annual contribution limit is 55,000 ($54,000 for 2017)