You would indeed want to consult previous (ideally recent with similar background therapies, as medical care may change over time) studies regarding what you would expect on placebo. If there are no previous similar trials, then non-trial data (e.g. dedicated observational databases/registries, electronic medical records databases, medical claims data etc.) could be alternatives. If hardly any data exists or its relevance is unclear, one can also try to elicit the judgments of a set of experts.
For the treatment group this would rather be an assumption and/or a number for which you want your study to have good power (e.g. a minimum important clinical difference, or perhaps something larger that constitutes a "great drug").
Also note, that given assumption about the placebo group survival time distribution (of course, just a median survival time does not define a whole distribution, a hazard ratio for drug vs. placebo and the assumption that the hazard ratio is constant over time, you already have the whole distribution for the treatment group.