How to incorporate varying time intervals between repeated measures in a General Linear Model repeated measure covariate analysis in SPSS, HELP!

Ok so I am sure there are some SPSS whizzes on here so please help me even if its you trying to remember and sending me in sort of the right direction…

So I have some data of yolk sac absorption of trout and I want to look at the rate of absorption taking into account varying egg sizes at the start of the trial. I have been told by my tutor to perform a General Linear Model analysis with egg size as a controlled variable however when I plot the graph and run the stats the model assumes that I have equal time intervals between my repeated measures, this is not the case. This obviously affects the curvature of my graph and therefore whether it follows a log/linear etc scale (as is my understanding). How can I incorporate the varying time intervals into my repeated measure analysis so that my graph shows this variation?
I hope this makes sense, I am rather confused myself!

Please note: I know how to do a general linear model covariate analysis, it is incorporating the varying time between the repeated measures I am having a problem with!